Stramenopila is a large group including parasites (e.g. opalinids), saprobes (e.g. oomycetes), phagotrophs (e.g. bicosoecids) in addition to the major aquatic producers (Heterokontophyta). They are basically unicellular but oomycetes (Oomycota) are composed of multinuclear mycelia and brown algae (Heterokontophyta) form large multicellular thalli with differenciated tissues. Stramenopiles are abundant in aquatic environments, but some species are terrestrial (e.g. on soil, parasite to land plants).

The flagellate cells possess two (anterior and posterior) flagella. The anterior flagellum is accompanied with tubular mastigonemes (origin of the name "strameno-pile"). The tubular mastigonemes make simple flagellar beat to produce backward current. In the transitional region of flagella usually includes transitional helis. Mitochondrial cristae are tubular. Heterokontophytes possess plastids surrounded by four membranes via secondary endosymbiosis with a red alga. Asexual reproduction by means of binary fission, zoosporogenesis, sporogenesis etc. Isogamy to oogamy are known in the sexual reproduction of stramenopiles. Although stramenopiles usually show diplontic life cycle (opalinids, oomycetes, diatoms, Raphidophyceae), the life cycle of brown algae is veried.