We describe and analyze a mixture model for supervised learning of pro
babilistic transducers. We devise an online learning algorithm that ef
ficiently infers the structure and estimates the parameters of each pr
obabilistic transducer in the mixture. Theoretical analysis and compar
ative simulations indicate that the learning algorithm tracks the best
transducer from an arbitrarily large (possibly infinite) pool of mode
ls. We also present an application of the model for inducing a noun ph
rase recognizer.