The green mussel Perna viridis (Linnaeus 1758), a native of the Indo-Pacific, has been introduced to the Atlantic basin and Caribbean Sea. It was reported first in Cienfuegos Bay, Cuba, in 2005. This species has proliferated quickly and now constitutes an economic as well as ecological problem, interfering with the operations of a local thermoelectric plant. The objectives of this study were to document the spatial and temporal variation in P. viridis density across Cienfuegos Bay and to examine the relationship between P. viridis density and water-quality patterns within the bay. The presence and relative abundance of P. viridis were surveyed qualitatively in Cienfuegos Bay in January 2011. Four sites were selected for additional investigation, and mussel density was estimated 6 times during 2011 and 2012. Temperature, salinity, and dissolved oxygen also were measured. We found that P. viridis density differed across Cienfuegos Bay both temporally and spatially, and that, although salinity may play a role in green mussel distribution and density, other factors were also important. The greatest densities of P. viridis were located near the city of Cienfuegos on the northeastern shore of the bay. The results of our systematic sampling also revealed dramatic temporal variation in the Cienfuegos Bay P. viridis population. During the 22-mo study, green mussel density at two of the sampling sites declined by an order of magnitude. There are several possible explanations for the dramatic decline in P. viridis density during the sampling period. Among them are mortality resulting from stressful environmental conditions and removal through harvest, both coupled with low recruitment rates. The decline of the population in the bay suggests that this species is not a candidate for a commercial fishery and that manual harvest may be an efficient means of eradication, if desired.