Researchers consistently report that children from low socioeconomic status (SES) families have, on average, smaller vocabularies as assessed by measures of existing vocabulary knowledge than children from higher SES families. Yet, few studies have examined the word-learning process of children from low SES families. The present study was an examination of fast mapping by preschoolers from low SES families. The study also examined the relation between measures of existing vocabulary and performance on the fast mapping task. Forty-six preschoolers (mean age: 4;6, range: 3;11-5;3) from low SES families completed a part-term fast mapping task and two measures of existing vocabulary knowledge. On the fast mapping task, children demonstrated the use of three sources of information (familiar whole objects, possessive syntax and whole-part juxtaposition). Measures of existing vocabulary did not correlate with performance on the fast mapping task. Findings suggest that children from low SES families use multiple sources of information in linguistic input to learn words. (Contains 5 tables, 1 figure, and 1 note.)