Relative to mainland Boa constrictor, boas from islands off the coast of Belize are described as being smaller, having longer tails, more elongate snouts, and proportionately larger eyes. However, no systematic confirmation of these patterns has been made. A morphometric study was initiated to investigate the body size and head shape variation between island and mainland boas in Belize. One hundred twenty-nine boas from five islands and the mainland were caught and measured. Univariate and multivariate analyses indicated that, in general, previous descriptions are accurate. Island boas are about half the length and one-fifth the mass of mainland boas. In contrast to mainland boas, no sexual size dimorphism is evident in island boas. The head shape of island boas differs from that of mainland boas but this divergence is not consistent among populations. Some island boas have more attenuate snouts compared to mainland boas whereas other island boas have larger eyes and narrower heads. Male island boas have longer tails compared to males from the mainland, but such a difference is not found in females from the two localities. The morphology of island boas is consistent with an arboreal habit and reduced prey size. Because these changes have occurred over an extremely short time interval, this may be another example of the speed and magnitude of adaptation that is possible in squamates.