The mutant stambhA1 (2-56.8) of Drosophila melanogaster was identified as a reversible temperature sensitive adult and larval paralytic. We have (i) isolated and analysed phenotypes of one new homozygous viable paralytic allele and two recessive unconditional embryonic lethal alleles of stmA and (ii) studied the interaction of the viable paralytic alleles with ts paralytic mutants napts1 (2-55.2) and parats1 (1-53.9). The homozygous viable paralytic alleles stmA2 and stmA1 are semi dominant neomorphs. The lethal alleles stmA12 and stmA7 appear to be amorphs. Unhatched embryos expressing lethal stmA alleles showed hypotrophy of the anterior dorsal cuticle overlying the brain with a concomitant hypertrophy of the anterior dorsal neurogenic region (the brain). The ventral cuticle was poorly differentiated, and the ventral nerve chord showed mild hypertrophy and poor organisation. The epidermal cells in 12-13 h old embryos did not show the normal palisade layer arrangement. These phenotypes are similar to mutant phenotypes of the neurogenic class of genes whose wild type functions are necessary for intercellular communication. The alleles stmA1 and stmA2 do not appear to interact with the paralytic mutants napts1 or parats1 in double mutant combinations. On the basis of our results it is proposed that stmA may belong to the neurogenic class of genes.