Diatom species diversity in the natural diet of healthy adult green abalone (Haliotis fulgens) and pink abalone (H. corrugata) from NW Mexico was analyzed and, referenced with values of condition index and hepato-gonadic index, used respectively as indicators of the health and reproductive season for the abalone. Likewise observations were done in abalone specimens affected with withering syndrome (WS). Our hypotheses stated that: a) the diatom assemblages represented in the gut contents of H. fulgens vs. H. corrugata would show a typical structure with few abundant taxa and many rare or uncommon species; b) no difference would be found between the structure of diatom assemblages in the gut contents of H. fulgens and H. corrugata; and c) diatom assemblages in abalone specimens with WS would be similar to those of healthy specimens. To test our hypotheses we examined healthy abalones collected from February 2102 to February 2013, and specimens with WS collected from March to May 2012. All specimens were measured and weighted and their condition and hepato-gonadic indices calculated. To analyze the diatom species diversity in gut contents of the abalone, specimens from 7 sampling dates from March to September 2012, were dissected. The diatom samples were described using species relative abundances, diversity indices and similarity between samples. For this period 19 diatom taxa comprised 80% of the relative abundances; the most abundant were Cocconeis scutellum, Gomphonemopsis pseudexigua, Grammatophora gibberula, Bacillaria socialis and Striatella unipunctata. High diversity values were estimated for diatom samples in all abalone specimens. Similarity and diversity values indicate that throughout this period both abalone species fed on the same diatom assemblages available in situ. Likewise, diatom samples in the guts of specimens affected by WS represented diatom assemblages similar to those of healthy specimens. No evidence was thus at hand for rejecting our hypotheses.