Two virus isolates from California sea lions and one from an Alaskan fur seal, classified as caliciviruses based on their relationship to vesicular exanthema of swine virus, were examined for biochemical and biophysical properties. They all had a buoyant density of 1.37 g/ml in CsCl; one showed some heterogeneity in CsCl. The sedimentation rate in 5–20% sucrose was 183S for all three. Crystals were obtained from one of the viral preparations. RNA, but no DNA, was present inall three. The molecular weight of the genome was probably 2.6 × 106 daltons, and the particle mass probably 12 × 106 dalt