Let θ(x) = (x − 1)/3 if x 1, and θ(x) = 2x/(1 − x) if 0 x < 1. We conjecture that the θ -orbit of every nonnegative rational number ends in 0. A weaker conjecture asserts that there are no positive rational fixed points for any map in the semigroup Λ generated by the maps 3x + 1 and x/(x + 2). In this paper, we prove that the asymptotic density of the set of maps in Λ that have rational fixed points is zero. Moreover, we prove that certain types of elements in the semigroup Λ cannot have rational fixed points.