Virus-like particles (VLPs) measuring 385±5 nm in diameter are described in the freshwater dinoflagellateGymnodinium uberrimum. The VLPs are found in association with, and “budding” from a vesicular viroplasmic area. A similar viroplasm was also found in a chrysophycean alga,Mallomonas sp. collected from the same general area in Saginaw Bay of Lake Huron. The nature of these VLPs and their virogenic stroma, in these algae from the Laurentian Great Lakes are discussed in the present report.