Survival ofLegionella pneumophila SG 1 in seawater and river water was assessed using plate counts on buffered charcoal yeast extract agar amended with α-ketoglutarate (BCYEα) and [(3)H]thymidine-labeling. The [(3)H]thymidine-labeling method for assessing survival ofL. pneumophila in aquatic environments was compared with viable counts, direct fluorescent microscopy (DFA), and acridine orange direct counts (AODC). Protozoa were isolated from the samples employed in the study and identified by characteristic trophozite and cyst morphology. Selective filtration employing 2.0 μm Nucleopore filters was used to determine the effect of grazing on survival ofL. pneumophila in seawater and river water.Legionella viability as measured by plate counts (CFU/ml), declined to a greater extent than cell lysis, assessed by thymidine, DFA, and AODC counts, suggesting thatL. pneumophila survives in aquatic habitats to a greater extent than revealed through culturable counts.