The objective of the study was to determine the infectious origin of the alevines mortality and lessions observed in adults of Oncorhynchus mykiss Walbaum 1792, "rainbow trout", in the fishfarm " El Ingenio" using a simple and effective metodology. Alevines, eggs and adult forms and samples of water from ponds were collected These samples were cultivated on seeds of Cucurbita maxima "calabash" as substrate, where typical colonies had been developing after seven days. Microscopic characteristics of the hyphaes, fitted the graphic pattern of Saprolegnia sp. (fungi), which is related to the symptomatology observed in the captured adults. The presence of Saprolegnia sp. would be related to high mortality of alevines registred (40%), probably for importation of eggs contaminated with the fungi. The method was efficient because the substrate has permited the development of the fungi for an easy and cheaply application.