Daytime changes in the droplet effective radius (re) and cloud liquid water path (LWP) were examined over a large area of marine stratocumulus off the coast of California over a six-day period using Geostationary Operational Environmental Satellite (GOES) 9 imager measurements. Amplitude and phase of the first harmonic of the Fourier series were used to represent the daytime cycle. Complex spatial variation in the amplitude was found. Mean amplitudes were 0.65 &mgr;m (re) and 13.8 g m-2 (cloud LWP). For cloud LWP the cycle peaked predominantly in the morning while re maxima occurred in both morning and afternoon. While attention has focused on the re afternoon maximum, these observations show that in fact the morning maximum is more common and has a stronger cycle, underscoring the poorly understood nature of the diurnal cycle of marine stratocumulus microphysical properties. ¿ 1999 American Geophysical Union