Two insurance companies I1, 1 I2 with reserves R1(t), R2(t) compete for customers, such that in a suitable differential game the smaller company I2 with R2(0) < R1(0) aims at minimizing R1(t) 􀀀 R2(t) by using the premium p2 as control and the larger I1 at maximizing by using p1. Deductibles K1, K2 are fixed but may be different. If K1 > K2 and I2 is the leader choosing its premium
first, conditions for Stackelberg equilibrium are established. For gamma distributed rates of claim arrivals, explicit equilibrium premiums are obtained, and shown to depend on the running reserve difference. The analysis is based on the diffusion approximation to a standard Cramér-Lundberg risk process extended to allow investment in a risk-free asset.