Phytoplankton collected bimonthly by net during 1991-2 from the Sontecomapan lagoon (12 km long and 1.5 km wide), in the Gulf of Mexico, included four species and several forms of the diatom genus Skeletonema. The spatial and temporal distribution of Skeletonema subsalsum, S. pseudocostatum and S. costatum in the lagoon varied with the salinity. S. subsalsum was dominant in the rainy season (July-February), when salinity was 0-6 ppt. S. coslatum was abundant in the adjacent marine zone, occasionally with S. tropicum, and occurred in the lagoon during the dry season (March) at salinities of 3.2-37.0 ppt. S. pseudocostatum was present the whole year showing a preference for brackish conditions (4-25 ppt). Morphological variation in three species was observed by light microscopy and studied in detail by electron microscopy. The shape of the external tubes of the fultoportulae, considered as an important taxonomic character, Varied in S. subsalsum and S. pseudocostatum. We observed long threads in S. subsalsum and S. pseudocostatum, associated with unicellular or short-chained forms, probably to confer buoyancy. S. subsalsum, previously found only north of 40 degrees N, is here reported for tropical waters. This is the first record of S, subsalsum and S. pseudocostatum in Mexico.
