A sentinel chicken program for West Nile virus (WNV) surveillance was initiated in July 2006 in eastern Puerto Rico, yielding the first seroconversions on June 4, 2007. WNV was isolated from sentinel chicken serum and mosquito pools (Culex nigripalpus, Culex bahamensis) for the first time in Tropical America. Preliminary sequence analysis of the prM and E genes revealed a 1-amino acid difference (V159A) between the Puerto Rican 2007 and the NY99. This mutation has been observed in the current dominant clade circulating in the United States. Sentinel chicken surveillance was a useful tool for the detection of West Nile virus in the tropics.