ABSTRACT. Seven clonal isolates of viruses which cause lysis of the eukaryotic, naked, photosynthetic flagellate Micrornonas pusilla (Prasinophyceae) were isolated from the coastal waters of New York, Texas, California and British Columbia, as well as the oligotrophic waters of the central Gulf of Mexico. The viruses are large polyhedrons (ca 115 nm dia.) lacking tails, and are morphologically similar to a previously described virus (MPV) which lnfected M. pusilla. Restriction fragment analysis of the DNA from these clones using EcoRI revealed unique banding patterns, demonstrating that each of the clones (including 3 that were isolated from the same water sample) were genetically different. Summation of the 17 to 26 visible fragments from the restriction digests, for each of the clones, yielded estimated genome sizes of 77 to 110 kilobase pairs. In contrast. only 4 different types of viruses could b e recognized based on the molecular weights of the major proteins. In field samples the concentrations of vlruses causing lysis of M. pus~lla were found to be spatially and temporally variable, ranging from < 20 to 4.6 x 106 infective units I-' Our results demonstrate that thls genetically diverse but morphologically similar group of viruses is widespread in nature. As viral infections propagate rapidly when host density is high, the presence of these viruses may place constraints on the maximum density that M. pusilla could reach in nature. If viruses infecting other phytoplankton taxa are similarly w~despread then they are probably a major factor maintaining diversity in phytoplankton communities, and are also likely important players in nutrient and energy cycling.