Sponges are a prominent component of coral reef ecosystems. Like reef-building corals, some sponges have been reported to bleach and die. The giant barrel sponge Xestospongia muta is one of the largest and most important components of Caribbean coral reef communities. Tissues of X. muta contain cyanobacterial symbionts of the Synechococcus group. Two types of bleaching have been described: cyclic bleaching, from which sponges recover, and fatal bleaching, which usually results in sponge death. We quantified hsp70 gene expression as an indicator of stress in X. muta undergoing cyclic and fatal bleaching and in response to thermal and salinity variability in both field and laboratory settings. Chlorophyll a content of sponge tissue was estimated to determine whether hsp70 expression was related to cyanobacterial abundance. We found that fatally bleached sponge tissue presented significantly higher hsp70 gene expression, but cyclically bleached tissue did not, yet both cyclic and fatally bleached tissues had lower chlorophyll a concentrations than nonbleached tissue. These results corroborate field observations suggesting that cyclic bleaching is a temporary, nonstressful state, while fatal bleaching causes significant levels of stress, leading to mortality. Our results support the hypothesis that Synechococcus symbionts are commensals that provide no clear advantage to their sponge host. In laboratory experiments, sponge pieces incubated at 30 °C exhibited significantly higher hsp70 expression than control pieces after 1.5 h, with sponge mortality after less than 15 h. In contrast, sponges at different salinities were not significantly stressed after the same period of time. Stress associated with increasing seawater temperatures may result in declining sponge populations in coral reef ecosystems.