Photosynthetic responses to irradiance by the photosymbionts of the two Red Sea sponges Theonella swinhoei (Gray) and Cliona vastifica (Hancock) growing under dim light conditions were measured in situ (in September 1997) using a newly developed underwater pulse amplitude modulated (PAM) fluorometer. Relative rates of photosynthetic electron transport (ETR) were calculated as the effective quantum yield of photosystem II (Y) multiplied with the photosynthetic photon flux (PPF). Photosynthesis versus irradiance (P-I) curves, obtained within minutes, showed that individual specimens of both sponges, growing under very low light conditions, feature lower light saturation points as well as lower maximal ETRs than individuals growing under higher light. Evaluations of such curves using low irradiances of the actinic light source (20 to 130 μmol photons m-2 s-1) showed a general decrease in Y, with a shoulder from the lowest irradiance applied till 20 to 30 μmol photons m-2 s-1. Point measurements yielded ETRs close to what could be estimated from the P-I curves. These point measurements also revealed good correlations between the diurnally changing ambient irradiances (1 to 50 μmol photons m-2 s-1) and average ETR values for both species. Further analysis showed that although Y values varied considerably between the different point measurements, they did not decrease significantly with light under these very low irradiances. Therefore, PPF rather than Y seems to determine the in situ diel photosynthetic performance at the low ambient irradiances experienced by these sponges.