Spatial memory, the capacity to recall one's position in space as a result of prior experience with a location, is important for most vertebrates. Spatial learning is associated with the recollection of landmarks in an area. There has been extensive analysis of the nature of spatial memory in organisms such as mammals and birds, but less evolutionary derived organisms, such as fish, have received less attention and provide opportunities to address spatial memory. The relatively small size and abundance of certain fish allow studies at the population level, which is not the case for larger, less abundant organisms. For this study, two ecologically contrasting populations of threespine stickleback fish (Gasterosteus aculeatus) were used to investigate the relationship between different ecological niches and the shapes of specific regions of the brain, which recent behavioral studies have implicated in the capacity for spatial learning. A bottom-feeding (benthic) population and a plankton-feeding (limnetic) population were studied. Benthic threespine stickleback feed on large, relatively static organisms at the bottom of lakes, suggesting the importance of spatial learning capacity to remember where prey tend to be abundant and to return to those sites. The deep-bodied shape of benthics further suggests that they are able to move in spatially complex environments because their center of mass is located in an ideal position for turning. In contrast, limnetics feed on plankton, which float in open water and have unpredictable locations from day to day. The streamlined bodies characteristic of limnetics suggest a lifestyle for swimming long distances through the water with little change in direction. Because of these differences in how benthics and limnetics locate food, the shapes of their brains should differ. Seventy formalin-preserved threespine stickleback fish caught in 2003 from two Alaskan lakes were used: thirty-five from Mud Lake (benthic) and thirty-five from Beaverhouse Lake (limnetic). Each fish was preserved in separate vials in 10% buffered formalin (100% formalin ? 37% formaldehyde). Brains were carefully extracted from each fish and stored in a 1.5 mL vial filled with the 10% buffered formalin. Pictures of the dorsal side of each brain were taken using a digital camera mounted to a microscope. The shapes of the forebrain and optic tectum (midbrain) were analyzed using geometric morphometrics, a set of set powerful new statistical techniques that allows analysis and visualization of shape variations in biological structures. The data were analyzed using TPS Software, created by Dr. James Rohlf of Stony Brook University in 2001. The qualitative and quantitative interpretations made for a clear comparison of shapes of different regions of the brain.