Thalassiosira allenii is a potentially harmful marine diatom distributed along the Northern Aegean and Southern Black Sea coasts of Turkey. In order to better understand the effect of environmental factors on T. allenii, the effects of 6 different light intensities (6.5, 38.7, 77.5, 116.2, 15 and 193.7 mu mol/m(2)s (PAR)) and 4 different temperatures (4, 11, 16 and 20 degrees C) were investigated. T. allenii was isolated from Izmir Bay-Northern Aegean Sea-Turkey and incubated in a f/2 enrichment medium under constant light in a batch culture system. The growth rate of the species was measured as Chl a (mu g/l) by monitoring the exponential growth phase. The statistical analysis of the obtained data was performed by utilizing the method of least squares. As a result of the study, the light intensities of 0.8, 3.6, 13.4 and 26.4 mu mol/m(2)s were found to be saturation light intensities for 4, 11, 16 and 20 degrees C, respectively whereas a light intensity of 6.5 mu mol/m(2)s was determined to be the compensation light intensity at 20 degrees C. The temperatures below 11 degrees C were detected to limit the maximum growth rate (mu(max)). Under saturated light conditions between the temperatures 20 - 11 degrees C the algae maintained its mu(max) parameter while it reduced its half-saturation constant K-L and exhibited a shift from sun to shade type. It was concluded that the light intensity is more effective compared to temperature in excessive reproduction of the algae in its natural environment.