We have studied the influence of some mutations in the bithorax complex on the observed synapsis dependent phenotype of the genotypes Cbx1Ubx1/+ and bx3 4e/Ubx1. The effect of these mutations is similar to that introduced by disruption of pairing or by the za mutation. Among the bx mutations, we find that bx8 behaves differently from most other bx mutations in its influence on the synapsis dependent phenotype. This observation induced us to map the position of bx8 with respect to other bx mutations; we find that it maps between bx3 4e and bx3. We show how some of the observations reported here can be fitted into a model of activation of the bithorax complex proposed by one of us.