We have cloned and characterized the first non-vertebrate member of the myc gene family, pAv-myc, from testes of the Northern sea star, Asterias vulgaris (Echinodermata). We have used an oligonucleotide that is complementary to the virtually 100% conserved vertebrate c-myc box A in the second exon and a cDNA library constructed from spermatogenically active testes. Relatives of this echinoderm existed approximately 100 million years prior to the origin of the earliest known organism for which c-myc sequence is currently available (the trout). Nonetheless, our cDNA encodes a protein with approximately 30% amino acid identity and 46% overall conservation to human c-myc. Regions of substantially higher conservation (63-95%) correspond in order to the transcriptional activation (boxes A, B and C), casein kinase II phosphorylation, nuclear-targeting, basic DNA-binding and oligomerization domains in the second and third exons of human c-myc. Sea star c-myc cDNA detects a 2.7-kb transcript on Northern blots of monthly samples of testicular tissue from field-collected individuals (n = 6-8), indicating peak expression during active spermatogenesis. This is also the first example of seasonal variation in expression of c-myc during spermatogenesis in laboratory or natural populations of any animal. It is intriguing to speculate about the potential oncogenic character of c-myc in this invertebrate, in which tumors have not yet been observed, and about the possibility that c-myc is more widely present in eucaryotes than has been anticipated.