Examination of blood from randomly selected residents in four subdivisions of Puerto Limón, Costa Rica, demonstrated Wuchereria bancrofti infection in 1.8% of 2,879 persons. Previous studies in four other subdivisions of the city revealed infection in 2.7% of 2,338 persons. In both studies the microfilaremia rate in males was approximately twice that in females. Distribution by race was similar in the two surveys, with microfilaremia in 1% of 1,052 white and 4.1% of 1,286 black residents in the first survey and 0.8% of 2,216 white and 5.4% of 663 black residents in the second. The highest rate of infection was in the 10-to 19-year-old and over 40-year-old age groups. In 13 rural communities near Puerto Limón, infection was found in only three (0.1%) of 3,448 individuals (27% of the population). Dissection revealed filarial larvae in 164 (6%) of 2,714 female Culex pipiens fatigans captured in 137 of 140 urban houses and in none of 936 from rural houses. Precipitin tests on stomach contents of 643 C. p. fatigans showed that 85% had taken human blood. The results indicate that endemic bancroftian filariasis in the Puerto Limón area is restricted to the urban population.