This paper presents preliminary observations on the upstream massive migration of    freshwater fishes in two rivers located at the Pacific slope of Province of Coclé,    Panama. Upstream movements begin in November and finish approximately a month    later. Two orders, four families and eight freshwater fish species have been found on    these upstream movements. Among the families, the Characidae showed the higher    number of migratory species. Most of the migratory fishes were juveniles.