Herbicides are among the most detected pesticides in coastal environments. Herbicides may impact non-target organisms, but invertebrates that have a symbiotic relationship with microalgae (zooxanthellae) may be particularly susceptible. How zooxanthellae influence the response of organisms to herbicides, however, remains untested. We exposed zooxanthellate and azooxanthellate Cassiopea xamachana medusae to environmentally relevant concentrations of the herbicide atrazine (0 µg L - 1, 7 µg L - 1 and 27 µg L - 1) for 20 days. We hypothesised that atrazine would have adverse effects on the size, rate of bell contractions and, respiration of medusae, but that effects would be more severe in zooxanthellate than azooxanthellate medusae. We also predicted that photosynthetic efficiency, chlorophyll a (Chla) content and zooxanthellae density would decrease in zooxanthellate medusae exposed to atrazine. Both zooxanthellate and azooxanthellate medusae shrank, yet the size-specific respiration rates were not constant during the experiment. Photosynthetic efficiency of zooxanthellate medusae significantly decreased at 7 and 27 µgL-1 atrazine, but atrazine did not affect the Chla content or zooxanthellae density. Our results showed that even though atrazine inhibited photosynthesis, zooxanthellae were not expelled from the host. We conclude that the presence of zooxanthellae did not increase the susceptibility of C. xamachana medusae to atrazine.