Threshold coefficient of virulence ( σ ) and host population size ( H ) required for a horizontally transmitted mutant (parasite 2) to invade a population of vertically transmitted feminising parasites (parasite 1) at equilibrium prevalence. In zone A, parasite 1 excludes parasite 2 and so the optimal level of horizontal transmission (β*) is 0. In zone B, parasites 1 and 2 coexist stably when horizontal transmission of parasite 2 = β*. In zone C, parasite 2 invades and displaces parasite 1 when horizontal transmission of parasite 2 = β*. Parameters for parasite 1 are estimated from Nosema granulosis infecting Gammarus duebeni: ε = 0.84, θ = 0.66, g = 0.27, a = 2, β = 0, b0 = 0.12, s = 0.000042, y* = 0.747. Parameters for parasite 2 are identical to those of parasite 1 except that β > 0.