There are four criteria set forth in Koch's postulates which are essential to prove the causal organism of any disease in nature. These are: (1) association of an organism constantly in all the diseased specimens, (2) isolation of the disease causing organism and culturing it, (3) inoculating the isolated organism on healthy host and duplication of natural disease symptoms, and (4) reisolation of the causal organism from experimentally inoculated host and comparing it with the initially isolated organism. It is obligatory to establish these postulates with regard to zoosporic fungal parasites also. However, it is difficult to fulfill all the above mentioned criteria for the obligate zoosporic fungi which cannot be cultured in the routine laboratory media. In such instances, a detailed and careful examination of the disease symptoms and the endobiotic fungal parasites is to be recorded. Maintaining dual culture of the healthy and infected host also helps to fulfill these postulates partially.