Abdominal-B (Abd-B) is a homeotic selector gene required for the correct development of abdominal segments 5-9. This gene contains at least two different genetic sub-functions which act as 'morphogenetic' or 'regulatory' elements within a single lethal complementation group. In an effort to identify the molecular nature of these sub-functions we have characterized homeobox-containing transcripts of Abd-B. The Abd-B transcripts can be grouped into four major classes whose diversity arises through differential exon splicing. We have isolated cDNAs which represent three of the transcript classes, designated alpha, beta and gamma. Their expression patterns, coupled with previous molecular genetic analyses of Abd-B, suggest that the alpha transcript class encodes the 'morphogenetic' sub-function while the beta transcript class contributes to the 'regulatory' sub-function.