Ampeliscu holmesi is reported herein from the grass beds behind the barrier islands of the northern Gulf of Mexico. Previous records are reviewed and the male of the species is described for the first time. The material agrees well with the original description and the recent redescription. Mouthparts for A . holmesi are described, illustrated, and compared with those of A. verrilli from the northeastern coast of the United States. Ampelisca holmesi Pearse, 1908, was described from Ferguson’s Pass, Oyster Bay, Florida (Pearse 1908). This area corresponds with a region now known on most maps as Espero Bay on the southwestern coast of Florida, just south of Charlotte Harbor. An additional record for this species from the Gulf of Mexico is Pearse (1912), who examined material collected by the ALBATROSS off the Mississippi Delta from 50 to 54 meters. Several records for this taxon exist from the eastern coast of the United States. Shoemaker (1933, p. 3) cited the material in the collections of the U. S. National Museum and reported the distribution of the species to be “from m o d e Island; Connecticut; Beaufort, North Carolina; Key West, Florida; and Sarasota Bay, Florida.” Material examined during this study included four indivi-, duals collected from the northeastern Gulf of Mexico: two adult females, 12-13 mm, J. M. Gathof, collector, 25 October 1976 30”14’N, 88’18‘W; 1 adult male, 12 mm, G. D. Goeke, collector, 14 March 1983 30”15’N, 88”44‘W; 1 adult female, 10 mm, GCRL 167-794,27 October 1967, southem side of Little Deer Island, Mississippi, D. H. Farrell, collector. Two of the females were collected from Diplantha wrightii grass beds, 1 mile east of the northwestern tip of Dauphin Island,Alabama,in 1 meter, using a 12-cm-diameter, plunger-type marsh corer. The single male was collected from D. wrightii grass beds at the northwestern tip of Horn Island, Mississippi, in 1 meter, using a scallop dredge. Both sites were characterized by a medium-sand substrate with detrital grass fragments at the sediment-water interface. Many large, tube-dwelling polychaetes, Diopatra cuprea, were present at the Dauphin Island collection site. Ampelisca holmesi is very closely related to A . vem‘lli Mills, 1967, and the nature of this sibling species pair has caused some confusion in the records for the distribution of the former species. Mills (1967) has indicated that some of the records are almost certainly based on specimens of A. Manuscript received July 1,1983;accepted September 7,1983. verrilli, but was unable to confirm his suspicions as the material examined by Shoemaker (1933) could not be located in the holdings of tlie National Museum. Mills also indicated that other records from the eastern coast should be assigned to A . vem‘ZZi (see Mills 1967 for synonomy of A . verrilli). Additional records for the distribution of A. holmesi are the north central Gulf of Mexico (Farrell 1970) and the southwestern coast of Cuba (Ortiz 1978). Mills (1 967) listed the differences between A . holmesi and the closely related A. vem-ZZi and stated that increased collecting would probably show the species “to be two members of a species flock related in similar features of head and pereopod 5” (p. 639). This appears to be the situation, as collections from the eastern Gulf of Mexico have revealed the presence of three undescribed but closely related species which possess the same generalized head and leg shapes (Goeke and Heard, in preparation). The mouthparts of ampeliscids often are of specific diagnostic value (Goeke, unpublished data). A careful comparison of the mouthparts of A. holmesi and A . vem*lli from the type locality has shown only minor differences. In sibling species pairs, mouthparts generally agree very well in structure and such is the case herein. Minor differences in the setation on the mandibular palp, facial setae of the palp of maxilla 1 and the number of gill rakers may all be attributed to age or clinal variations within the species. While it is unfortunate that no substantive diagnostic features could be found in the mouthparts, it demonstrates well the close relationship between the two species. Maxilliped (Figure 1H) palp normal for the genus, without diagnostic features for the species; inner margin of outer plate armed with 10 chisel-shaped spines and 4 setal spines, each spine with accessory seta; inner plate with row of submarginal medial and terminal setae, terminal margin with 2 setal spines and 2 chisel-shaped teeth (Figure 1 I). Maxilla 1 (Figure 1 G) palp with 2 segments, 3 outer marginal plumose setae, 5 terminal spines and approximately 14 simple facial setae; outer plate with 11 terminal spines, the