Arbuscular mycorrhizal (AM) fungi enhance terrestrial plant growth by forming a symbiotic relationship with the roots of its host plant. Effects of red and green algal extracts on in vitro hyphal growth of AM fungi, Gigaspora margarita and Glomus caledonium, and on the mycorrhizal development and growth of papaya seedlings and passionfruit cuttings were investigated. Three kinds of red algae, Gracilaria verrucosa, Gelidium amansii and Eucheuma cottonii, and a green alga, Chlorella pyrenoidosa, were extracted with 75% methyl alcohol (MeOH), and these extracts were fractionated by a flash chromatograph equipped with an octa decyl silane column and eluted with 0, 10, 25, 50, and 100% MeOH. Twenty-five percent MeOH fractions (25% MeOH eluates) of each type of alga most significantly increased in vitro hyphal growth of AM fungi. Inhibition of AM hyphal growth was not observed in any of the fractioned MeOH eluates. Root colonization of papaya and of passionfruit were markedly stimulated by the application of the 25% MeOH eluates of red and green algal extracts. The growth of these plants were promoted in comparison to the control. These results suggest that red and green algae not only contain AM stimulatory substances, but also contribute to mycorrhizal development in higher plants.