There are over 40,000 small ranch ponds in Montana. More than threefourths of these were constructed solely for stock-water purposes and most of the others for either irrigation or the control of erosion. Fewer than 50 are known to have been built specifically for fish. (Brown and Thoreson, 1951). While approximately 700 ponds have been stocked with fish of one species or another, less than 100 are known to have produced any appreciable fishing. During 1949 and 1950, the Montana Agricultural Experiment Station and the Montana State Fish and Game Department conducted a cooperative investigation of existing Montana ranch ponds to explore their suitability for fish. About 20 ponds scattered over the eastern (plains) part of the state were examined at regular intervals throughout all seasons and inventories were made on a large number of other ponds as time permitted. The data included certain physical, chemical and biological information considered essential to an understanding of fish production. A summary of the physical-chemical information collected on twelve ponds is given in Table 1. The most striking physical feature is the prevailing low temperature even during the summer period. The water in ponds of this area is generally quite alkaline with the pH as great as 10.0 and total alkalinities (methyl orange) as high as 325 ppm. While aquatic plants of varied species are present in many ponds, they have not been found detrimental in ponds which were deep enough to insure fish production. In one small pond, mortality of trout was definitely attributed to low dissolved oxygen resulting from duck weed (Lemna) which completely covered the surface and which was found in large quantities, decaying on the bottom. Past records show that the following fish species have been planted in Monta a ponds: rainbow trout (Salmo gairdnerii), brook trout (Salvelinus fontinalis), brown trout (Salmo trutta), cutthroat trout (Salmo clarkii lewisii), bl egills (Lepomis machrochirus), largemouth bass (Micropterus salmoides), black crappie (Pomoxis nigro-maculatus), y llow perch (Perca flavescens), walleye (Stizostedion vitreum), northern pike (Esox lucius), black bullheads (Ameiurus melas), golden shiner (Notemigonus crysoleucas), and bluntnose minnow (Hyborhynchus notatus).