Abstract: Deuring gave a now classical formula for the number of supersingular elliptic curves in characteristic p. We generalize this to a formula for the cycle class of the supersingular locus in the moduli space of principally polarized abelian varieties of given dimension g in chacteristic p. The formula determines the class up to a multiple and shows that it lies in the tautological ring. We also give the multiple for g up to 4. This is joint work with S. Harashita.