Feline calicivirus (FCV) F9 strain was propagated in Crandall-Reese feline kidney cells. Two density populations of viral particles were observed after equilibrium centrifugation in an isopyknic CsCl gradient. The buoyant density of the heavy particle (PH) is 1.33 g/ml. The light particle (PL), a previously undescribed form of feline calicivirus, has a buoyant density of 1.22 g/ml. The PH and PL presented a similar morphology by electron microscopy. Western blot showed that both PH and PL contained a major polypeptide of the typical FCV capsid protein with a molecular weight of 62,000. Infectivity assay and RNA isolation demonstrated that PH is the intact infectious virion while PL is FCV empty capsid.