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Because of the short incubation period of most acute infectious diseases, short-term and daily mobility are more important than permanent and seasonal migration for the spread of these diseases. Yet most studies of population mobility focus on permanent or semipermanent change of residence. Here, we describe the results from a field study conducted on the island of Dominica, West Indies, during the summers of 1989 and 1990 and the winter of 1991. The study was designed to collect data on short-term mobility rather than migration. These mobility data are linked with data on the patterns of measles transmission during a 1984 epidemic. Three-hundred five individuals from all parts of the island were interviewed about their daily travel patterns, their travel off the island, and the travel of members of their immediate family. In addition to these respondents, interviews were conducted with representatives of most of the major occupations that involve travel in the course of a workday. Data were also collected on the number and type of motor vehicles traveling along various routes on the island and on travel of native residents to the capital city, Roseau, to buy or sell at the major weekly market. Analysis of the interviews shows that travel within the island is clearly nonrandom. For example, almost everyone interviewed traveled to Roseau at least once a month, but 40% of the respondents had never been to any of the major villages in the Grand Bay Health District, which is only about a half-hour from Roseau. Patterns of disease transmission have been directly affected by these mobility patterns. The measles epidemic in 1984 apparently did not reach the Grand Bay Health District, even though all other areas of the island experienced significant rates of infection. Analysis of reasons for the relative isolation of the Grand Bay Health District indicates the importance of transportation patterns, as well as social, cultural, and geographic factors, to the disease transmission patterns throughout the island.