An interesting phenomenon in human speech perception is the trading relation, in which two different acoustic cues both signal the same phonetic percept. The present study compared American English, Spanish, and monkey listeners in their perception of the trading relation between gap duration and Fl transition onset frequency in a synthetic say-stay continuum. For all the subjects, increased gap duration caused perception to change from say to stay; however, subjects differed in the extent to which the Fl cue traded with gap duration. For American English listeners, a change from a low to a high Fl onset caused a phoneme boundary shift of 26 msec toward shorter gap durations, indicating a strong trading relation. For Spanish listeners, the shift was significantly smaller at 13.7 msec, indicating a weaker trading relation. For monkeys, there was no shift at all, indicating no trading relation. These results provide evidence that the say-stay trading relation is dependent on perceptual learning from linguistic exposure.There is currently much interest in determining the basis for trading relations in human speech perception. A trading relation is a phenomenon in which two different acoustic cues signal the same phonetic percept. Within limits, the cues trade with one another along a synthetic speech continuum-that is, the value ofone cue may be increased while the value of the other is decreased, yet the phonetic percept remains the same for the listener. For example, one ofthe first trading relations to be studied in detail was that between voice onset time (VaT, a temporal cue) and Fl onset frequency (a spectral cue) as perceived by American English (AE) listeners: As the Fl onset frequency increases (decreases) in a synthetic VaT continuum, more voiceless (voiced) stimuli are perceived, thus shifting the phoneme boundary toward lower (higher) values of VaT (Summerfield & Haggard, 1977).What causes a trading relation? The most parsimonious explanation is that the effect is due to generalized psychoacoustic mechanisms that are innate and immutable in all normal listeners and, thus, independent oflanguage learning (e.g., Kluender & Lotto, 1994). A second possibility is that trading relations are due to perceptual learning from linguistic exposure, including articulatory processes (e.g., Best, 1995 nisms are not necessarily mutually exclusive, however, and it is likely that trading relations, as well as other aspects of speech perception, implicate both innate and learned mechanisms that interact to varying degrees (see, e.g., Pisoni, Lively, & Logan, 1994).There are several ways in which the relative contributions of psychoacoustic versus perceptual learning processes in speech perception could be evaluated. First, we can compare the perception of native and nonnative listeners. Differences would obviously support a perceptuallearning component, because there is no reason to expect psychoacoustic differences in sensitivity between native and nonnative listeners. Second, we can compare the perception ofnative list...