We have examined the rates of development of six wild-type and rapid-development strains of Dictyostelium discoideum. Strains NC4 and HU1231, a derivative of V12, have very similar developmental profiles. In comparison to these two strains, amoebae of the rapid-development strain HT100 reach the aggregation stage about 6 h earlier. Also, intracellular cAMP levels in HT100 increase precociously during early development. Postaggregative morphogenesis of HT100 proceeds at the same rate as NC4 and HU1231. In addition we have assessed the progress of development by RNA hybridization blotting. The appearance in HT100 of mRNAs associated with aggregation is advanced by 6 h while the appearance of early gene products is not affected appreciably. These data suggest that HT100 differs from strains NC4 and HU1231 primarily in the process of aggregation. We have partially characterized the genetic background of strain HT100. Mating and cell fusion analyses suggest that HT100 is a derivative of V12. We constructed a growth-temperature-sensitive derivative of HT100, and fused it with HU1231 cells. The resulting diploids develop at the same rate as HT100, suggesting that the rapid-development phenotype in HT100 is dominant.