An important area of investigation in the field of entrepreneurship examines how people and organizations exploit technological opportunities. Prior research suggests that alliances, the mobility of experts, and the informal mechanisms associated with geographic co-location can present firms with useful opportunities to source technological knowledge. This paper uses insights from strategic management and organizational theory to suggest that organizational size may have an important impact on the extent of external learning, since it differentially affects the likelihood of learning via formal and informal mechanisms. Examining a cross-section of semiconductor startups, we find that external learning increases with startup size. With regard to the specific mechanisms of learning, we find that firms learn from alliances regardless of their size. For the informal mechanisms of mobility and geographic co-location, however, learning decreases with firm size. These results suggest that as startups grow, they may have increasing opportunities to access and exploit external knowledge, but their motivation (and hence ability) to learn from more informal sources may decrease.