A simulation demonstrating the interaction of two feminising parasites and their joint effects upon the density of a host population. Parasite 2 is capable of both vertical and horizontal transmission (ε = 0.7, θ = 0.2, α = 0.02, β = 0.0002) but is unable to invade a host population (a = 2.3, g = 0.5, b0 = 0.12, s = 0.000042) at its uninfected equilibrium size of H* = 714. Parasite 1 is incapable of horizontal transmission but has more efficient vertical transmission and feminisation (ε = 0.75, θ = 0.5, α = 0.001, β = 0) and so is able to invade an uninfected population. Addition of a single female infected with parasite 1 at generation 400 results in the rapid spread of parasite 1 to equilibrium prevalence (y* = 0.25), increasing the equilibrium population size (H*) to 4113. This increase in population size allows parasite 2 to invade and achieve a prevalence of 0.36, driving the prevalence of parasite 1 down to 0.16.