Any of various nonphotosynthetic protists belonging to the phylum Oomycota and living in marine, freshwater, and soil environments. Oomycetes have cell walls made of cellulose or similar substances, and store their food as glycogen. They reproduce asexually by the formation of diploid spores (called zoospores) with two flagellae. Sexual reproduction involves the formation of a number of eggs within a structure called an oogonium. The eggs are fertilized by antheridia that penetrate the oogonium and deliver nuclei to the eggs. The oomycetes were formerly classified among the fungi because their filamentous bodies resembled fungal hyphae. The phylum includes several species that cause important plant diseases, such as late blight.