Ultraweak photon emission of dark-incubated A. acetabulum cells were measured with the use of a sensitive electronphotomultiplier of the Hamamatsu 550 type. The photon count series were subjected to Fourier analysis for 2-1020 sec period range. The average level of photon emission in samples containing 50 cells was approximately. 40% above background. Cell cultures were prepared at least 24 hr before the photon emission measurements and kept un-disturbed ("established cultures"). This paper reports results of Fourier analysis of a number of samples of Acetabularia cells. In a single population cells periodicity of light emission was not defined directly from Fourier transformation. A large number of analyses, however, if they are combined and compared with background data, reveal a cell-culture specific frequency pattern. The results suggest the idea that established cell-cultures are characterized by higher intensities of long period (minutes) oscillations occurs, while a relative decrease was observed in the short period (few seconds) range. The long period oscillations were not detected in cell cultures that were prepared within 1 hr before the photon emission measurements. It is concluded that Fourier analysis of ultraweak photon emission, even with relatively low signals, appears to be possible. It may serve as a non-invasive tool for monitoring the physiological state of cells, or for studying the control of intercellular dynamics.