diff --git a/codec/h265/lex.go b/codec/h265/lex.go
new file mode 100644
index 00000000..7baaf734
--- /dev/null
+++ b/codec/h265/lex.go
@@ -0,0 +1,157 @@
+/*
+NAME
+  lex.go
+
+DESCRIPTION
+  lex.go provides a lexer for taking h265 rtp format and lexing into access units.
+
+AUTHORS
+  Saxon A. Nelson-Milton <saxon@ausocean.org>
+
+LICENSE
+  Copyright (C) 2019 the Australian Ocean Lab (AusOcean).
+
+  It is free software: you can redistribute it and/or modify them
+  under the terms of the GNU General Public License as published by the
+  Free Software Foundation, either version 3 of the License, or (at your
+  option) any later version.
+
+  It is distributed in the hope that it will be useful, but WITHOUT
+  ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+  FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
+ for more details.
+
+  You should have received a copy of the GNU General Public License
+  in gpl.txt.  If not, see http://www.gnu.org/licenses.
+*/
+
+package h265
+
+import (
+	"encoding/binary"
+	"fmt"
+	"io"
+	"time"
+
+	"bitbucket.org/ausocean/av/protocol/rtp"
+)
+
+// NALU types.
+const (
+	typeAggregation   = 48
+	typeFragmentation = 49
+	typePACI          = 50
+)
+
+// Buffer sizes.
+const (
+	maxAUSize  = 100000
+	maxRTPSize = 4096
+)
+
+// Lexer is an H265 lexer.
+type Lexer struct {
+	UsingDON   bool            // Indicates whether DONL and DOND will be used for in RTP stream.
+	buf        [maxAUSize]byte // Holds current access unit.
+	off        int             // Holds offset into buf that we're occupying with access unit.
+	fragmented bool            // Indicates if we're currently dealing with a fragmentation unit.
+}
+
+// Lex continually reads RTP packets from the io.Reader src and lexes into
+// access units which are written to the io.Writer dst. Lex expects that for
+// each read from src, a single RTP packet is received.
+func (l *Lexer) Lex(dst io.Writer, src io.Reader, delay time.Duration) error {
+	buf := make([]byte, maxRTPSize)
+	for {
+		n, err := src.Read(buf)
+		switch err {
+		case nil:
+		case io.EOF:
+			continue
+		default:
+			return fmt.Errorf("source read error: %v\n", err)
+		}
+
+		// Get payload from RTP packet.
+		payload, err := rtp.Payload(buf[:n])
+		if err != nil {
+			return fmt.Errorf("could not get rtp payload, failed with err: %v\n", err)
+		}
+
+		nalType := (buf[0] >> 1) & 0x3f
+
+		// If not currently fragmented then we ignore current write
+		if l.fragmented && nalType != typeFragmentation {
+			l.off = 0
+			continue
+		}
+
+		switch nalType {
+		case typeAggregation:
+			l.handleAggregation(payload)
+		case typeFragmentation:
+			l.handleFragmentation(payload)
+		case typePACI:
+			l.handlePACI(payload)
+		default:
+			l.write(payload)
+		}
+
+		m, err := rtp.Marker(buf[:n])
+		if err != nil {
+			return fmt.Errorf("could not get marker bit, failed with err: %v\n", err)
+		}
+
+		if m {
+			_, err := dst.Write(l.buf[:l.off])
+			if err != nil {
+				// TODO: work out what to do here.
+			}
+			l.off = 0
+		}
+	}
+	return nil
+}
+
+// handleAggregation parses NAL units from an aggregation packet and writing
+// them to the Lexers buffer buf.
+func (l *Lexer) handleAggregation(d []byte) {
+	idx := 2
+	for idx < len(d) {
+		if l.UsingDON {
+			switch idx {
+			case 2:
+				idx += 2
+			default:
+				idx += 1
+			}
+		}
+		size := int(binary.BigEndian.Uint16(d[idx:]))
+		idx += 2
+		nalu := d[idx : idx+size]
+		idx += size
+		l.write(nalu)
+	}
+}
+
+// handleFragmentation parses NAL units from fragmentation packets and writes
+// them to the Lexer's buf.
+func (l *Lexer) handleFragmentation(d []byte) {
+
+}
+
+// handlePACI will handl PACI packets
+//
+// TODO: complete this
+func (l *Lexer) handlePACI(d []byte) {
+	panic("unsupported nal type")
+}
+
+// write writes a NAL unit to the Lexers buf in byte stream format using the
+// start code.
+func (l *Lexer) write(d []byte) {
+	const startCode = "\x00\x00\x00\x01"
+	copy(l.buf[l.off:], []byte(startCode))
+	copy(l.buf[l.off+4:], d)
+	l.off += len(d) + 4
+}
diff --git a/protocol/rtp/parse.go b/protocol/rtp/parse.go
index d658aa20..b60af30a 100644
--- a/protocol/rtp/parse.go
+++ b/protocol/rtp/parse.go
@@ -34,6 +34,19 @@ import (
 
 const badVer = "incompatible RTP version"
 
+// Marker returns the state of the RTP marker bit, and an error if parsing fails.
+func Marker(d []byte) (bool, error) {
+	if len(d) < defaultHeadSize {
+		panic("invalid RTP packet length")
+	}
+
+	if version(d) != rtpVer {
+		return false, errors.New(badVer)
+	}
+
+	return d[1]&0x80 == 1, nil
+}
+
 // Payload returns the payload from an RTP packet provided the version is
 // compatible, otherwise an error is returned.
 func Payload(d []byte) ([]byte, error) {