Targeted gene disruption was used to investigate the function of MYO1, an unconventional myosin gene in Tetrahymena thermophila. Phenotypic analysis of a transformed strain that lacked a functional MYO1 gene was conducted at both 20 degrees C and 35 degrees C. At either temperature the delta MYO1 strain had a smaller cytoplasm/nucleus ratio than wild type. At 20 degrees C, delta MYO1 populations had a longer doubling time than wild type, lower saturation density, and a reduced rate of food vacuole formation. However, at 35 degrees C, these characteristics were comparable to wild type. Although micronuclear division and cytokinesis appeared normal in delta MYO1 cells, failure of the macronucleus to elongate properly resulted in unequal segregation of macronuclear DNA in cells maintained at either 20 degrees C or 35 degrees C.