We studied, in a deterministic model, the gene flow between two morphs characterized by the alternative forms of a prezygotic reproductive isolating mechanism with monogenic inheritance. Incomplete reproductive isolation is due to the fact that the different genotypes mate in partially different sites or periods. The model is costituted by two populations which initially are monomorphic for the two alleles of the locus which causes reproductive isolation and for the two alleles of a neutral unlinked gene. After the contact, there is a constant symmetrical migration rate between the two populations. We have observed that the reduction of gene flow for the neutral gene, for a given intrinsic structure of mechanism (value of $z$ parameter), is a function of the migration rate : at high migration rates, there is a detectable reduction (which is strong only for very high $z$ values) ; at low migration rates, the effect is insignificant for all the z values tested.