Tetrahymena pyriformissyngen 1 was kept in the ultradian growth mode and synchronized by a sequence of 4‐h 20°C/l‐h 0°C cycles. Upon transfer to constant 20°C, the cells showed a free‐running ca. 5‐h rhythm of tyrosine aminotransferase activity. This rhythmicity persisted after treatment with cycloheximide applied in concentrations leading to inhibition of cell growth. These data suggest the existence of an ultradian oscillator, which seems to be independent or to be at least not the consequence of the cell cycle, but which might be related to the rhythmicity of cell division in normal conditions.