The green crab (Carduus maenas L.) is an exotic species in many parts of the world. It successfully invaded, at the end of the 1990s, the coast of Prince Edward Island, Canada, where rock crabs (Cancer irroratus Say) are commonly found. The relationship between these 2 species is still not well documented. The short-term time budget of both species was studied in autumn 2006 and spring 2007 under different experimental conditions to assess the effect of the green crab on the behavior of the rock crab. Crabs were observed in 3 competition treatments (1 crab, 2 conspecifics, or 2 heterospecifics) using 3 temperatures (5, 12, or 20°C) and 2 prey densities (4 or 30 mussels). The time spent foraging and in agonistic interactions by both species generally increased in high-temperature challenges, whereas the time spent inactive decreased, regardless of the season. Both crab species responded differently when facing a conspecific. Time spent foraging increased in certain cases whereas it decreased in others. Overall, the time budget of the green crab was only affected by temperature during autumn observations whereas the time budget of the rock crab varied according to (1) the competition treatment, (2) temperature, and (3) number of prey available. Temperature influenced the time budget of both crab species during springtime. The behavior of both species was unaffected by the presence of a heterospecific. These results suggest that rock crabs, in contrast to what was expected, are not much affected by the presence of green crabs and that they may coexist in nature.