In a discrete-trials procedure, a frequency-dependent schedule shaped left-right choice proportion toward various equilibrium values between 0 and 1. At issue was (1) whether pigeons match when the overall reinforcement probabilities for two responses depend inversely on their recent frequency, and (2)how pigeons meet the schedule constraint in terms of local responding. That is, do they respond quasi-randomly (Bernoulli mode), or do they learn the stable pattern of the schedule (stable-pattern mode)? Molar choice behavior always tracked the equilibrium solution ofthe schedule, but the molecular response patterns varied substantially. Markov chains applied to the data revealed that responding was generally intermediate between the memoryless Bernoulli mode, and the perfect memory stable-pattern mode. The polymorphism of molecular patterns, despite molar regularities in behavior, suggests that (1) in order to engender the Bernoulli or stable-pattern modes, the reinforcement rule must strongly discourage competing response patterns (e.g., perseveration), and (2) under frequency-dependent schedules, molar matching is apparently not the outcome of momentary maximizing.Laboratory studies have shownthat frequency-dependent selection, the differential reinforcement of infrequent responses, sometimes engenders high degrees of behavioral variability. For example, when Page and Neuringer (1985) rewarded pigeons for generating sequences of eight left or right pecks that differed from the sequences generated on the last 50 trials, they obtained highly variable, random-like behavior. That is, the birds not only pecked each key equally often, but also showed few or no sequential dependencies between consecutive responses (see also Hest, Haaren, & Van De Poll, 1989;Morgan & Neuringer, 1990;Machado, 1989Machado, , 1992Morris, 1987Morris, , 1989Neuringer, 1991Neuringer, , 1992. However, frequencydependent selection may also engender stereotypical behavior. In a previous study (Machado, 1992), a frequencydependent schedule differentially reinforced the momentarily least likely of two responses-the more a pigeon preferred the right key, the less food it received from that key and the more food it received from the left key. Similar contingencies operated when preference shifted to the left; the probability of food for left (L) and right (R) pecks was equal only at indifference. Although the pigeons pecked each key on 50% of the trials, they did so by alternating rather than responding randomly.The preceding results illustrate two points. First, pigeons can track the overall probabilities of reinforcement for each of two responses when these probabilities I thank John Staddon and the other L.A.B. group members for their comments on earlier versions of the manuscript. The research was supported by grants to Duke University from NSF and NIMH (John Staddon, principal investigator). Correspondence concerning this article may be sent to A. Machado,