Engorged female ticks of Boophilus microplus were exposed in wire-gauze cylinders and glass tubes in an experimental grass plot at monthly intervals during 1989, and egg laying, egg development and larval survival periods observed and recorded. Rainfall and atmospheric relative humidity had an important influence on tick activity. Egg production was maximum, hatching percentage was high, incubation and prehatch periods were short, and larval survival and total longevity periods were long for ticks exposed during the warm and humid rainy season from June to September. Dry atmospheric conditions severely affected egg development, egg hatch and larval survival. Eggs failed to hatch in the dry months from December to April and only 29-38% hatched after a long incubation period of 41 days in November and May. On grass, the larvae of ticks exposed in November survived for the shortest period of 28 days and the larvae of ticks exposed in June survived for the longest period of 133 days. Low winter temperatures reduced egg production and prolonged the pre-oviposition, oviposition and incubation periods. It is suggested that the results of this study might be helpful in the development of measures to control tick infestation by planned dipping and restricted grazing during the period from late June to January when the pasture has a substantial load of larval ticks.