Recent molecular studies on dreissenid species in southern Europe found that Dreissena blanci (Westerlund, 1890) resides in several lakes where previous studies identified it as Dreissena polymorpha (Pallas, 1771). Because D. blanci seems to constitute a “new record” in the Balkans, the present study provides novel information on the biology and ecology of its planktonic larvae in four natural lakes and one reservoir located in western Greece. Dreissena blend larvae were present in all lakes and their abundance varied between 3.4 and 440 ind. I-1. The larvae were present all year round, having greater abundance in spring and summer and lesser in winter. The species seems to have an extended reproductive period from early spring until late summer. The larvae are distributed mainly in the upper 20 m of the deep lakes Amvrakia and Trichonis, while are aggregated close to or within the thermocline layer in the latter lake. A size-specific depth distribution was observed, with larger larvae residing in deeper strata during the stratification period. Temperature appears to be the most important parameter affecting the abundance variation of the larvae in the natural lakes, while water retention time is the major parameter in Stratos reservoir. In contrast, the soft, muddy substratum of Lake Lysimachia seems to be the inhibiting factor for the existence of a viable population here. The great similarities in larval ecology between D. blanci and D. polymorpha must be taken into consideration, as D. blanci could become another invasive species in freshwater ecosystems. Therefore, more research is required on the life cycle of this species, and the ecological and economic consequences of its presence in lakes and reservoirs of southeastern Europe.