We present the results of a study of the Lepidoptera species inhabiting the various biotic zones of the western slope of the Sierra Nevada Mountains, from the Great Central Valley to the mountain crest, in an area around an approximate transect from the city of Fresno to Mono Pass. The investigation methods included day-catching, day- and nocturnal trapping, searching and rearing of immature stages, and catch and release techniques. We identified 1092 species. Most of them represented Noctuoidea, Geometroidea, Gelechioidea, Pyraloidea, and Tortricoidea superfamilies. Most species were encountered at the base of the mountain, in the Foothill Woodland zone, indicating a low-plateau pattern of species richness for our sampled area. We found a bimodal annual species richness curve at the lower elevations (Valley Floor and Foothill Woodland) with a peak in May/June and another one in October. At higher altitudes there was only one richness peak, in July. In the Valley Floor and Foothill Woodland zones there was continuous flight activity throughout the year, while at higher altitudes this activity became gradually restricted to the warmer periods of the year. Anthropogenic changes were detected in the natural habitat at all elevations. These were most pronounced in the Valley Floor and Foothill Woodland zones, thereafter decreasing with the altitude. We found a direct relationship between the mean low temperature values in any given zone and month and the number of species flying at that time and altitude. We assessed this relationship visually, from the charts we plotted for each zone with the pertinent values. During this study, we encountered larval-host plant associations for Udea vacunalis (Pyralidae), Anoncia mosa (Cosmopterygidae), and Aristotelia sp. (Gelechiidae) for which we could not find any references in the literature as well as certain larval habits of Plebejus acmon and Agriades podarce (both Lycaenidae) that expand the awareness concerning their larval behaviors.