This paper presents an adaptive control strategy for a class of first-order nonlinear systems of the form x = theta1*T f(x) + theta2*T g(x) g(x) is a smooth function, whereas f(x) satisfies sectoricity conditions. theta1* and theta2* are constant parameter vectors. It is assumed that the system remains controllable for all values of x, but the sign of theta2*T g(x) is unknown. The proposed adaptive scheme extends ideas previously presented in [1] where the term premultiplying the input was supposed to be constant. The standard least-squares estimates of theta2* are modified using a hysteresis type switching algorithm that enables us to conclude on existence, uniqueness, boundedness and convergence of the solutions of the adaptive closed-loop system.