Linguistic parallelism (i.e., the tendency of similar forms to occur together within a stretch of discourse) has been shown to be very strong in many linguistic phenomena and in a vast number of languages. Examining the role of phrase-level parallelism on noun phrase number agreement, this article demonstrates that Puerto Rican Spanish and Brazilian Portuguese exhibit more similarities than differences with regard to this constraint. A detailed analysis of Brazilian Portuguese data is presented, and the results are compared with those found by Poplack (1980a) for Puerto Rican Spanish. Guy's (1981a) and Labov's (1994) hypothesis about missing zeroes in Brazilian Portuguese is also discussed. In conclusion, it is claimed that the phrase-level parallelism effect on noun phrase number agreement is embedded in a universal principle of linguistic use: parallel processing.