The purpose of this investigation is to present a new account of the genus Saprolegnia, to bring together in one paper the published studies dealing with the morphology and taxonomy of the various species, and to add to this information conclusions based upon observations of living material. The majority of living specimens was obtained from 864 collections of water and soil made by the writer and others in eleven states and in Australia, England and India.

The procedures used in the isolation and propagation of these fungi is presented. All isolates studied were propagated on a chemically defined medium, as well as, on halves of sterilized hemp seed (Cannabis sativa).

Reliable taxonomic criteria are used in the keys provided for the identification of species.

As a result of this investigation, fifteen species are recognized as valid members. Three new combinations are proposed. Four species are doubtful and eighteen taxa are excluded from the genus.