The Portobello mushrooms are the mature form of various brown coloured strains of the species. The different strains are different in more than just colour, but taste as well

About The Portabella  Mushroom "Agaricus bisporus"

Portobello mushrooms, common mushrooms, button mushrooms, white mushrooms, swiss brown mushrooms, roman brown mushrooms, cremini mushrooms are all the same "species" of mushroom. But the Portabella mushroom is one of the most imposing mushrooms in size. It has a rounded, earthen tan flat cap with near black gills on its underside. Its stem is thick, white and edible. The Portabella mushroom has a very memorable, rich and meaty texture and flavor, which it retains even after cooking.