_Catoptrophorus semipalmatus inornatus_ (Brewster): Willet.--A flock of 15 or 20 individuals was seen almost every day of our stay on the beach 8-13 km. WSW of Sisal. Measurements of two of the specimens are large and they are clearly referable to _C. s. inornatus_. Specimens (3): [Male], 40441, testis 3 mm., 242.3 gm., wing 205 mm., tail 80 mm., tarsus 66 mm., culmen (from feathers) 60 mm. (August 9), [Female], 40440, ovary inactive, 240.5 gm., wing 204 mm., tail 77 mm., tarsus 69 mm., culmen (from feathers) 59 mm. (August 11), and [Male], 40442, imm., 221.1 gm. (August 11) 8-13 km. WSW Sisal. Mallophaga: _Austrominopon sachlebeni_, August 9; _Quadraceps carrikeri_, August 11. _Arenaria interpres_ (Linnaeus): Turnstone.--Six individuals were sighted on Isla Mujeres on August 1 and one individual was sighted 4 km. W Sisal on August 11. _Crocethia alba_ (Pallas): Sanderling.--This species was relatively common at Sisal where several flocks of 30 to 40 birds were seen. One specimen (40444) shows an unusual pattern of molt; replacement of outer primaries has preceded that of the inner. Specimens (4): [Male], 40445, testis 3 × 1 mm., 57.5 gm., heavy fat (August 9), [Female], 40444, ovary granular 6 × 3 mm., 56.7 gm., molting (August 11), [Male], 40443, 35.5 gm. (August 11), and [Female], 40446, imm., 54.1 gm. (August 11), 6-1/2-13 km. WSW Sisal. Mallophaga: _Lunaceps holophaeus actophilus_, August 11; _Carduiceps zonarius_, August 11. _Ereunetes pusillus_ (Linnaeus): Semipalmated Sandpiper.--The only individual seen was taken from a mixed flock of Sanderlings and Wilson Plovers. Specimen (1): [Male], 40447, testis minute, 24.4 gm. (August 11), 8 km. WSW Sisal. _Himantopus mexicanus_ (Müller): Black-necked Stilt.--A lone individual was sighted on Isla Mujeres on August 2. A pair was on the mudflats among mangroves at Sisal, of which the male was taken on August 7. Although Paynter (1955a) indicated that this species probably breeds on the peninsula, there are no definite breeding records. Our specimen had only slightly enlarged testes, but its close association with a female suggested that the pair may have been mated. Specimen (1): [Male], 40448, testis 7 × 3 mm., 189.4 gm., little fat (August 7), 5 mi. SW Sisal. _Larus atricilla_ Linnaeus: Laughing Gull.--This species was seen regularly along the coasts of the peninsula. Specimen (1): [Female], 40449, ovary 7 mm. (inactive), 253 gm. (August 7), 4-3/10 mi. W Sisal. Mallophaga: _Quadraceps_ sp. _Sterna albifrons antillarum_ (Lesson): Least Tern.--A flock of about 20 individuals was seen on Isla Mujeres; two individuals were shot and appear to be the first specimens from the island. Specimens (2): [Female], 40451, ovary slightly enlarged, 49.0 gm. (August 2); sex? 40450, 41.8 gm. (August 2), Isla Mujeres.