Two haploid cell lines have been established from androgenetic embryos of the frog, Rana pipiens; one line has been maintained in culture for 150 generations, the other for 200 generations. Karyotypes of the two lines agree well with the standard for the species although some chromosomes show small differences in length. The cells multiply in the same defined basal medium used for culture of other anuran cell lines; this medium consists of the usual amino acids, vitamins, and serum macromolecules plus an exogenous purine source. Both the haploids resemble „permanent” cell lines in their prolonged multiplication in culture. The two lines differ in their mode of growth, one being epithelial-like, the other forming an overlapping meshwork of fibroblast-like cells. Both have the low plating efficiency characteristic of „unaltered” cells. These two lines are exceptional in their ability to compete successfully with the diploid variants which arise by endomitosis or cell fusion and which usually overgrow the haploid population. The more vigorous line, RPH 68.2A, should provide the long-desired haploid material for genetic studies in cell culture.