Counting spermatocyte bivalents, we have determined the haploid numbern= 16 forAplysia depilansandA. punctata, andn= 17 forA. limacina.We have confirmedn= 17 forBursatella leachii.Male diakinetic bivalents are probably chiasmatic in all the species examined. The orders Nudibranchia and Notaspidea, possessing haploid values of 13 and 12 chromosomes respectively, seem to be more evolved than the other orders of the sub-class Opisthobranchia.