Chlorella pyrenoidosa grown under low (100 foot-candles) and high light intensity (1050 foot-candles) exhibit marked changes in electron spin resonance signals. The ratio of the light-induced fast rise, fast decay signal (signal I) to the broader, slow decay signal (signal II) is greater in the low-light grown cells. The ratios of paramagnetic ion signals apparently due to ferric (Fe 3+) ion and manganous (Mn 2+) ion are also greater in the low-light grown cells. Electron micrographs show that the most salient change in cell structure of the cells grown under low light is a marked increase in the number and density of the lamellae. The effect of heat treatment of cells on cell structure and electron spin resonance signals also is discussed.