Fus3, the mitogen-activated protein kinase (MAPK) of the mating pheromone response pathway, inhibits a post-translational step of Ty1 retrotransposition. Fus3 also inhibits haploid invasive growth by blocking cross-activation of invasive growth gene expression by the pheromone response signal cascade. Here, we show that Fus3 kinase activity and dosage co-ordinately regulate Ty1 transposition and invasive growth. A chromosomal copy of the kinase-defective fus3-K42R allele fails to inhibit either Ty1 transposition or invasive growth. When overexpressed, kinase-defective Fus3 weakly inhibits both Ty1 transposition and invasive growth, but is much less inhibitory than wild-type Fus3 expressed at the same level. Moreover, increasing the dosage of wild-type Fus3 intensifies the inhibition of both Ty1 transposition and invasive growth. To demonstrate that Fus3 regulates Ty1 transposition via its negative regulation of the invasive growth pathway, we show by epistatic analysis that the invasive growth pathway transcription factors Ste12 and Tec1 are both required for Fus3-mediated inhibition of Ty1 transposition. When haploid invasive growth is stimulated by high-copy expression of TEC1, by expression of the dominant hypermorphic allele STE11-4 or by deletion of HOG1, Ty1 transposition is concomitantly activated. In summary, these results demonstrate that the haploid invasive growth pathway activates Ty1 transposition at both transcriptional and post-transcriptional levels and that Fus3 inhibits Ty1 transposition by inhibiting the invasive growth pathway.