A conditional maximum likelihood model is used to estimate the size-selectivity of trawls, gillnets, and hooks when the data are obtained by simultaneous fishing with meshes or hooks of different size and/or shape. Size-selectivity is expressed here by the selection curve,r(l), the probability that a fish of lengthl, if contacting the gear, will be retained (caught). In many selectivity studiesr(l) is fitted either by eye, by heuristic means, or by improper application of generalized linear models. Then it is not possible to make legitimate statistical inference aboutr(l), or about assessments of the state of the fishery if those assessments user(l). It is shown here that by conditioning on the total catch, selectivity data can be modeled as binary data, or polytomous data on interval scales. Application of the model to trawl and hook data demonstrates that selection curves can be fitted using generalized linear models, which may require nonstandard link functions or link functions with parameters.