The present work is based primarily on a collection of fishes made in Laos by the author in the year 1970-1971 as a part of the fisheries development project sponsored by U.S. AID Mission to Laos, and to a lesser extent on a collection made there by the author in 1969. All species derived from these collections, 146 in number, are fully described and illustrated. This catalogue also deals with published accounts pertaining to Laos by recent authors, i.e., Fowler (1934a, 1935b, 1937), Pellegrin and Fang (1940), Smith (1945), Serrene (1951), and Taki (1968), from which 57 species are cited. The citation includes a number of fishes collected in Thailand at several places on the Mekong River where the flow is shared by Laos and Thailand. These fishes taken in the Thai-Lao Mekong, although they have been included in Thai fish fauna, are regarded as inhabitants of Laotian water as well as of Thai. Thus the present publication reports altogether 203 species falling into 31 families from the Lao Mekong basin.