This paper combines records from the United States census with records from the Puerto Rican census and compares for the first time the fertility levels of Puerto Ricans who migrated to the United States with those of their counterparts who remained in Puerto Rico. The general hypothesis tested is that migration and residence in a place where low fertility is the norm results in lower fertility among the migrants than among those who remained at the place of origin. In general, the effect of migration to the mainland is considered to reduce fertility, but to a very small extent. Some evidence that this effect of migration on fertility is diminishing is stated to exist. Results of the analysis of the relationship between the ability to speak English and current fertility for urban island residents is held to indicate that those who speak English have considerably lower levels of current fertility than those who do not. The ability to speak English is seen to be an indicator of a whole cluster of attitudes and behaviors associated with what is labeled "modernism." The importance of micro-census data when addressing the issue of migration and fertility is stressed. (Author/AM)