In controlled laboratory transmission experiments, uniform doses of axenic in vitro isolate cultures of Perkinsus marinus from a Crassostrea virginica oyster, and of independent P. chesapeaki isolates from Chesapeake Bay Mya arenaria and Macoma balthica clams, were used to reciprocally challenge Perkinsus sp.-free C. virginica, M. arenaria, and M. balthica experimental hosts. Following mantle cavity inoculations, all 3 experimental hosts acquired high incidences (30 to 100%) of infections by each of the 3 Perkinsus sp. isolates, based on PCR assays of DNAs from experimental host tissues that were collected through 60 d post-inoculation. Lesions containing proliferating pathogen cells were documented histologically in tissues of all experimental host species challenged with all isolates of both Perkinsus species. Experimental Perkinsus sp. challenge isolates were re-isolated and propagated in vitro from infected tissues of host molluscs from most (5 of 9) experimental treatment groups. Koch's postulates were generally satisfied to confirm experimental infections in all bivalve molluscs that were challenged with 3 isolates of 2 Perkinsus spp. These results suggest potential broad and overlapping host specificities for the 2 current Chesapeake Bay-endemic Perkinsus species: P. marinus and P. chesapeaki.