A monthly study on subtidal marine algae from Bajo Pepito, Isla Mujeres, Quintana Roo, Mexico, was conducted from February 1997–January 1998. We identified 166 taxa including 76 Rhodophyta, 61 Chlorophyta, and 29 Phaeophyta. A considerable monthly replacement of species was evident during this period, indicative of a dynamic algal community. Macroalgae occupied about 33% of the available substrata. Species of Rhodophyta had the greatest average cover, with 20.5%, followed by Phaeophyta with 7.1%, and Chlorophyta with 5.4%. Greatest cover, by genus, was by the brown alga Dictyota (6.1%). Hydropuntia crassissima (P. Crouan & H. Crouan in Schramm & Mazé) M. J. Wynne and Hydropuntia cornea (J. Agardh) M. J. Wynne two agarophytes, were probably affected by the rest of the algae community: smaller biomass of those two species was related to the greater cover of the other macroalgae. Conspicuous species exhibited more than one maximum value in size and in the number of reproductive plants. Differences in phenology among the species likely allow their coexistence. We report plantules as means of vegetative reproduction in Dictyota mertensii (Martius) Kuetzing and Lobophora variegata (Lamouroux Womersley).