  We study a contact process (CP) with two species that interact in a symbiotic
manner. In our model, each site of a lattice may be vacant or host individuals
of species A and/or B; multiple occupancy by the same species is prohibited.
Symbiosis is represented by a reduced death rate, mu < 1, for individuals at
sites with both species present. Otherwise, the dynamics is that of the basic
CP, with creation (at vacant neighbor sites) at rate lambda and death of
(isolated) individuals at a rate of unity. Mean-field theory and Monte Carlo
simulation show that the critical creation rate, lambda_c (mu), is a decreasing
function of mu, even though a single-species population must go extinct for
lambda < lambda_c(1), the critical point of the basic CP. Extensive simulations
yield results for critical behavior that are compatible with the directed
percolation (DP) universality class, but with unusually strong corrections to
scaling. A field-theoretical argument supports the conclusion of DP critical
behavior. We obtain similar results for a CP with creation at second-neighbor
sites and enhanced survival at first neighbors, in the form of an annihilation
rate that decreases with the number of occupied first neighbors.
