Oxygen and pH microelectrodes were used to investigate the microenvironment of the planktonic foraminifer Orbulina universa and its dinoflagellate endosymbionts. A diffusive boundary layer surrounds the foraminiferal shell and limits the O₂ and proton transport from the shell to the ambient seawater and vice versa. Due to symbiont photosynthesis, high O₂ concentrations of up to 206% air saturation and a pH of up to 8.8, i.e. 0.5 pH units above ambient seawater, were measured at the shell surface of the foraminifer at saturating irradiances. The respiration of the host-symbiont system in darkness decreased the O₂ concentration at the shell surface to < 70% of the oxygen content in the surrounding air-saturated water. The pH at the shell surface dropped to 7.9 in darkness. We measured a mean gross photosynthetic rate of 8.5 ± 4.0 nmol O₂ h^-1 foraminifer^-1. The net photosynthesis averaged 5.3 ± 2.7 nmol O₂ h^-1. In the light, the calculated respiration rates reached 3.9 ± 1.9 nmol O₂ h^-1, whereas the dark respiration rates were significantly lower (1.7 ± 0.7 nmol O₂ h^-1). Experimental light-dark cycles demonstrated a very dynamic response of the symbionts to changing light conditions. Gross photosynthesis versus scalar irradiance curves (P vs E(o) curves) showed light saturation irradiances (E(k)) of 75 and 137 μmol photons m^-2 s^-1 in two O. universa specimens, respectively. No inhibition of photosynthesis was observed at irradiance levels up to 700 μmol photons m^-2, s^-1. The light compensation point of the symbiotic association was 50 μmol photons m^-2 s^-1. Radial profile measurements of scalar irradiance (E(o)) inside the foraminifera showed a slight increase at the shell surface up to 105% of the incident irradiance (E(d)).