Published online by Cambridge University Press: 17 April 2009
Finite O-simple semigroups with a Rees representation over an elementary abelian group are considered. The isomorphism classes of such semigroups with class graph γ and coefficient group of order pd are shown to correspond to the orbits under the automorphism group of γ of those subspaces of the circuit space of γ over Zp which have codimension at most d. A semigroup interpretation of the inclusion relation between subspaces is given and results are obtained on the structure of the automorphism group and the enumeration of the semigroups under consideration.