In abalone farms and laboratories of the Baja California peninsula in Mexico, young and postlarval abalone feed on benthic diatoms that grow on the artificial surfaces of the culture tanks, or on epiphytic diatoms growing on kelp (Macrocystis pyrifera) blades provided when a change in their diet is estimated. It is not known whether grazing selectivity occurs by postlarvae and young abalones at this time. Thus, experiments were conducted to determine whether discrimination by juveniles of red abalone (Haliotis rufescens) occurs while grazing on diatoms that grow on the rocks and kelp blades extracted from their habitat, and to determine whether the ingested diatoms are, indeed, digested. Our hypothesis was that juveniles of H. rufescens would feed nonselectively on the diatom films; thus, the characteristics of the diatom assemblages (species composition, diversity) from their gut contents should be similar to those upon which they feed. We also expected that the smaller forms of diatoms would pass through the digestive tract intact. Overall, 171 diatom taxa were identified: 123 in the tank walls, 35 on the rocks, 143 in the gut contents, and 38 in the feces. Only Amphora pediculus appeared in all the gut contents examined; however, 12 taxa accounted for 80% of the relative abundance (n = 500). Highest diversity ( = 4.46) was measured in the walls of a tank where M. pyrifera was included, but abalone specimens from this tank showed the lowest H' value in their gut contents. The hypothesis that grazing by young H. rufescens is nonselective is supported, as well as the importance of the structure of the diatom assemblage grazed upon (as in H. fulgens), which is reflected in their gut contents. Moreover, feces analysis showed that many small diatoms that are ingested are not digested, and are able to reproduce and grow after passing through the digestive tract.