We tested the ability of different integrin alpha subunits to substitute for each other during embryonic development. Two alpha subunits, which form heterodimers with the same betaPS subunit, are expressed in complementary tissues in the Drosophila embryo, with alphaPS1 expressed in the epidermis and endoderm, and alphaPS2 expressed in the mesoderm. As a result the two integrin heterodimers are present on opposite surfaces at sites of interaction between the mesoderm and the other cell layers where they are required for normal development. Using the GAL4 system, we are able to rescue fully the embryonic lethality of an alphaPS2 null mutation with a UAS-alphaPS2 transgene, but only partially with a UAS-alphaPS1 gene, due to partial rescue of both muscle and midgut phenotypes. Similarly we are able to rescue the embryonic/first instar larval lethality of an alphaPS1 null mutation gene with UAS-alphaPS1, but only partially with UAS-alphaPS2. Each UAS-alpha gene, when it contains the cytoplasmic domain from the other alpha subunit, maintains an equivalent ability to rescue its own mutation and cannot fully rescue a mutation in the other alpha. We conclude that the two alpha subunits are not equivalent and have distinct functions which reside in the extracellular domains.