The method of Wetherall (1986, Fishbyte 4(1):12-14) for estimating Z/K and L sub( infinity ) from length-frequency data sampled from a population in equilibrium (constant recruitment and mortality) was examined through simulation of population experiencing rapid change in recruitment and fishing mortality. The results show a large bias of Z/K and L sub( infinity ) to be generated under non-equilibrium situation; this bias cannot be removed by smoothing. However, departure from equilibrium can be detected, and hence the conditions identified where themethod remains applicable.