Differentially expressed integrins have been shown to be involved in the intricate cell movements that occur during early development. Because the migration and movement of cells have been well characterized in sea urchin embryos, we searched for alpha-integrin subunits in this organism. An alpha integrin subunit, alphaP, was cloned from Strongylocentrotus purpuratus mesenchyme blastula stage mRNA by RT-PCR and RACE and found to exhibit 74-77% sequence similarity to mammalian alpha(5), alpha(8), alpha(IIb), and alpha(v) integrin. The 8-kb transcript was most abundant at the prism stage, although low levels could be detected at all stages by Northern blot analysis and RT-PCR. A polyclonal antibody to this novel integrin was generated against a 100-amino-acid alphaP fragment fused to glutathione S-transferase and shown to recognize a 180-kDa alpha-integrin in the egg and in all stages of embryogenesis studied.