Localization of Tetrahymena profilin was examined by an immunofluorescence method. In interphase Tetrahymena cells, immunofluorescence for profilin was diffusely distributed in the cytoplasm, while in dividing cells, additional intense fluorescence was observed in the division furrow. From the result of immunofluorescence localization using cytoskeletal cell models, a significant fraction of profilin appeared to become insoluble in association with a cytoskeletal structure just beneath the division furrow during cytokinesis, although remaining profilin existed as a soluble form in the cytoplasm. Double immunofluorescence staining with anti-profilin and anti-actin antibodies revealed that the localization of profilin in the division furrow coincided with that of contractile ring microfilaments in terms of both position and timing. This is the first report describing the coexistence of profilin with actin filaments in the division furrow, implying the possible involvement of profilin in assembly and disassembly of contractile ring microfilaments in the process of cytokinesis.