A bismuth or bismuth alloy plating bath comprises as essential ingredients an organic sulfonic acid of the general formula (I)
wherein R is a C 1 - 5 alkyl radical, X 1 is a halogen atom or hydroxyl, aryl, alkylaryl, carboxyl, or sulfonyl radical which may be in any optional position of the alkyl radical, and n is an integer of 0 to 3, or of the formula (II)
wherein X 2 is a halogen atom or hydroxyl, alkyl, aryl, alkylaryl, aldehyde, carboxyl, nitro, mercapto, sulfonyl, or amino radical, or two X 2 ' S which may combine with a benzene ring to form a naphthalene ring, and m is an integer of 0 to 3, and a bismuth salt of the acid or a mixture of the bismuth salt of the acid and one or more other metal salts of the acid.