The dissimilation of sequential aspirated obstruents is investigated in Tangkhul Naga. Acoustic, aerodynamic, and perceptual data were gathered to understand the alternation between aspirated and unaspirated prefixes in the verbal morphology of the language. Production data confirm that prefix onsets are aspirated only when the onset of the following syllable is sonorous. Two prefixes may occur in sequence. The traditional account predicts that the first of these will be unaspirated because it precedes an obstruent, not a sonorant. This is confirmed in the production data. However, perceptual data suggest that speakers of the language tend to hear an aspirated consonant in this position. This strengthens the claim that processing of repeated items is difficult and perhaps dispreferred–a psychoacoustic explanation for phenomena such as Grassmann’s Law. This dissimilation pattern is still only perceptual in Tangkhul, but variation in production suggests dissimilation may be emerging at the production level also.