Amblyomma variegatum tick infestation, tick infection by Ehrlichia ruminantium (ER), and ER genetic diversity were studied in the Caribbean Islands of Guadeloupe, Marie-Galante, and Antigua between 2003 and 2005. Nested PCR for pCS20 was used to detect ER, while ER strains were characterized by sequencing or by restriction fragment length polymorphism (RFLP) profiles of map-1 PCR products. In 2003 in Guadeloupe, the prevalence of tick-infested herds was 35.6%. In Marie-Galante 79.1% of herds in 2003 and 73.8% in 2005 were infested, while only an average of 2.2% of the herds were infected in Antigua between this same period. In Marie-Galante, 19.1% of ticks were ER positive, and ER-infected ticks were found in 33.3% of the herds. In Antigua only 5.8% of the ticks were ER positive. High ER tick infection rate combined with a very high level of tick infestation highlight the risk of heartwater in Marie-Galante and Guadeloupe more than in Antigua. The three islands still represent a reservoir for tick and heartwater in the Caribbean. Nine different African and Caribbean map-1 ER genotypes were identified. This diversity was observed even in restricted areas, and identical map-1 genotypes were observed on all three islands. This high genetic diversity of ER strains suggests that there was a simultaneous introduction of several strains from African countries into the Caribbean region.