We prove some properties of the first eigenvalue for the elliptic system − Δ p u = λ | u | α | v | β v in Ω , − Δ q v = λ | u | α | v | β u in Ω , ( u , v ) ∈ W 0 1 , p ( Ω ) × W 0 1 , q ( Ω ) . In particular, the first eigenvalue is shown to be simple. Moreover, the stability with respect to ( p , q ) is established.