The annual reproductive cycle of Creagrutus guanes (Teleostei, Characidae) was studied in a tropical mountain river, a type of habitat where reproductive biology of fishes has been scarcely investigated. Analysis was made on adults, 70 males and 135 females, which were captured in 13 monthly samplings. The analysis was based on macro- and microscopic observations of gonads, and on weight of gonads, mesenteric fat, and liver. Weight and microscopic observation of ovaries and testes, as well as macroscopic observation of ovaries, suggest reproductive activity was concentrated in periods of low rainfall. In males macroscopic observations did not reflect the maturity state revealed by histology. Fat reserves also showed a seasonal variation related with rainfall pattern. Given that dry season reproduction is relatively uncommon in tropical freshwater fishes, ultimate factors determining such a pattern are discussed.