Diatom assemblages from an abalone (rocky) habitat were sampled in April and November 1999 and in April 2000 on the western side of Isla Magdalena, B.C.S., México. Overall 236 taxa were recorded, including 10 new records, and 56 species that have been observed exclusively in this type of habitat in the Baja California peninsula. The rocky habitat surveyed is much more complex than expected because of different substrata (rock, fleshy macroalgae, crustose corallines, erect corallines) available for colonization by diatoms at Isla Magdalena. Although epilithic forms were identified, epiphytic diatoms were more abundant. Thus the potential diet for abalone and other grazers is more diverse than previously assumed (i.e., that mainly epilithic diatoms would be their potential food source). A variation in structure was observed between the two assemblages sampled in April because of a change in the species composition of the samples. Most of the rock surface was covered by macroalgae. Thus, the diatom associations consisted mainly of epiphytic forms. The high values of H' corresponded to high species richness (S), whereas higher dominance (λ) corresponded to low S. The highest estimated value of H' was 5.39 (S = 82) for the November 1999 rock-Lithophyllum assemblage. Similarity measurements, using Morisita's index, indicate that differences in species composition and in association structure may represent a distribution of diatom taxa according to available substrata within the habitat rather than a year-to-year or seasonal variation.