The spontaneous rosette formation of peripheral leucocytes of carp with red blood cells of various mammals was investigated. Spontaneous rosette formation was observed with SRBCs, HRBCs and BRBCs with the frequency of rosette formation being higher with SRBCs and HRBCs than with BRBCs. Variations in surface morphology, in cell size of the rosette forming cells and in rosette formations were observed under light and scanning electron microscope. However, by examining Giemsa-stained preparates, the RFCs were identified as lymphocytes. The spontaneous rosette formation occurred gradually depend-ing on the incubation time. The spontaneous rosette formation was not specific to SRBCs and was observed at a temperature range of 0°C to 37°C. In these respects the rosette formation of carp lymphocytes differed from the earlier reported rosette formation of mam-malian lymphocytes.