In Experiment 1, 6 adults with mild mental retardation were taught 3 overlapping conditional discriminations in a linear series structure, establishing the possibility of the emergence of 2 stimulus equivalence classes of 4 stimuli per class. Training employed balanced trial types in which the discriminative stimuli were presented in fixed pairs across conditional discriminations (e.g., B1 and B2, C1 and C2). No participant showed the emergence of equivalence classes in initial testing; 1 participant showed the establishment of classes with repeated testing, including tests for symmetry alone. In Experiment 2, 6 additional adults with mild mental retardation were similarly trained, with one methodological modification: Prior to testing, each comparison stimulus (e.g., B1) was presented in some trials with every stimulus from the opposing class (i.e., A2, B2, C2, and D2). With this modification, 4 of 6 participants showed the establishment of equivalence classes during initial testing. The results support hypotheses about the essential role of simple discrimination acquisition in equivalence class establishment.