Marine Tubificidae (Annelida, Oligochaeta) from Bahia de San Quintin, Baja California, are described, and their distribution in the bay noted relative to the mean particle size of the sediments. Tubifex postcapillatus, n. sp. is characterized by its simple-pointed dorsal setae, and short hair setae which occur in postclitellar segments only. The setal pattern and details of the genital anatomy distinguish Thalassodrilus belli, n. sp. from other members of its genus. Both of these species are associated with the finest sediments which are located mainly at the north end of the bay. Limnodriloides monothecus, n. sp. with its single, mid-dorsal spermatheca unique to this genus, occurs mainly in silts and very fine sands (4 to 5 ø). Limnodriloides verrucosus, n. sp. possesses a papillate body wall previously unknown in the genus, but reminiscent of the Peloscolex condition; it occurs in all sediment types in the bay (3 to 8 ø) but is most abundant in the 5 to 6 range. The dominant oligochaete. Limnodriloides barnardi, n. sp., closely resembles Limnodriloides winckelmanni Michaelsen, especially in its possession of elongate, grooved spermathecal setae enclosed in vacuolated sacs; the species are distinguished by details of their penial structures. Limnodriloides barnardi, n. sp. occurs in all sediment classes but its optimum range appears to be in 4 to 6 ø sediments.