ABSTRACT Benthic foraminifers that host algal symbionts are similar to corals in that they rely on their algal endosymbionts for their energy needs, calcify prolifically, and are sensitive to changes in environmental conditions. They are abundant in the benthos of coastal coral-reef areas and are found throughout the tropical and subtropical regions. Pulse Amplitude Modulated (PAM) chlorophyll fluorometry and chlorophyll a extraction techniques were used to quantify and compare the photosynthetic responses of the benthic foraminiferal, Archaias angulatus and their isolated endosymbionts, Chlamydomonas hedleyi, to short-term changes in temperature. Maximum quantum efficiency (Fv/Fm) and rapid light curves (RLCs), from which relative electron transport rates (rETR) of photosystem II (PSII) were derived, were investigated over a thermal range from 4.4° to 33.9 °C in three experiments that were 7 to 31 days in duration. Typical mean yields (Fv/Fm for healthy holobionts (symbionts in hospite) were 0.6 - 0.7, and for isolated symbionts 0.5 - 0.6. Chronic photoinhibition, indicated by significant decreases in Fv/Fm, occurred at temperatures above 31.0°C; there was minimal reduction in efficiency in cooler treatments. The trends between holobiont and symbionts were very similar in all of the photophysiological parameters measured [yield, photoefficiency (<α>), ETRmax and minimum saturating irradiance (Ek)] and supported the temperature range findings in terms of the tolerance of the specimens in the low temperatures up to 31.0 °C. For all photochemical measurements assessed, the holobiont values tended to be somewhat higher than those for the symbionts, with the exception of Ek, possibly indicating a tight coupling in the host-symbiont response during photosynthesis. Chlorophyll a (<μ>g/foram) was negatively correlated with temperature (r = -0.37, p < 0.001) in Experiments 1 and 2. However, in all 3 experiments, chlorophyll a was variable, suggesting a high degree of individual variability in A. angulatus and the ability to acclimate. Some differences observed among treatments may be related to differences in seasons when the specimens were collected and in length of time in culture prior to experiments. Holobiont median rETR light curve trends and photophysiological derived parameters recorded median Ek ranges of ~100-150 <μ>mol photons m-2 s-1, observed ETRmax light intensities of ~200 <μ>mol photons m-2 s-1 and photoinhibition, induced by increasing irradiance intensities, which occurred > 500 <μ>mol photons m-2 s-1. These light curve trends and derived parameters generally supported previous photosynthesis O2 and CO2 gas production studies of A. angulatus. The differences in responses associated with acclimation should be considered in design of future experimental studies. This was the first known physiological study of the viable temperature range and photobiology of A. angulatus using chlorophyll fluorometry methods. Though commonly found in Caribbean and Atlantic waters ranging from 14.0 - 31.0 °C, these results indicate a wider thermal-tolerance range for A. angulatus than was previously known. Keywords: Foraminifera, Chlamydomonas sp., PAM fluorometry, photosynthesis, algal symbiosis