Cognates have served as a useful tool for investigating the bilingual lexicon in many studies, but very little research has been carried out on different types of cognates, specifically, partial cognates and their role in cross-linguistic effect. The present study examines cognate effect in the speech production and acceptability judgment of two groups of highly proficient, late-onset English-Spanish (n = 12) and Spanish-English (n = 12) bilinguals within a single-language (English) context. The findings of two tasks, a production task, whereby participants were asked to spontaneously produce synonyms to prompt words, and an acceptability judgment task of a variety of sentences including use of false and partial English-Spanish cognates are reported here, framed within non-selective, integrated models of lexical representation. The results suggest a significant cognate effect in both bilingual groups in both tasks compared to their monolingual counterparts with, surprisingly, greater significance demonstrated from L2 to L1 influence, particularly in production. These findings add to the growing support for semantic modulation at the conceptual level of lexical processing in highly proficient bilinguals.