Observations were made on vertical migration patterns of Haemonchus contortus infective larvae on Cynodon dactylon (bermudagrass) and Paspalum notatum (bahiagrass) pastures under summer climatic conditions typical of East Texas. Ten thousand H. contortus infective larvae (L3) were introduced to 100 cm(2) subplots of each pasture species within a plot area of 1m(2). Subplots were inoculated with larvae by applying them in an aqueous medium to the soil or mat beneath the vegetation. Herbage from the inoculated areas was harvested on 5 sampling days over a span of 21 days. L3 recoveries were observed and recorded each day on four herbage strata viz. 0-5, 5-10, 10-20 and >20 cm from ground level. The log transformed larval recovery data were analyzed for effect of day, stratum, and day x stratum interaction for each grass species during two separate experimental periods. Precipitation, relative humidity and temperature during the study were subjected to correlation and multiple regression analyses with the larval counts. Significant (P</=0.0017) differences were found for the effect of day on larval recoveries. No significant differences were detected for stratum or day x stratum interaction effects, though stratum provided a strong indication of influencing larval recovery. A high positive correlation (>or=0.93) between rainfall and total average daily larval counts was apparent. The multiple regression analysis did not show significant results for any of the climatic factors tested. This study showed that the H. contortus infective larvae can survive beyond 21 days in the soil and infest pasture grasses when the climatic conditions are favorable. Avoiding use of H. contortus contaminated pasturelands in summer at the onset of rainfall following a dry spell may effectively reduce nematode loads in susceptible farm animals. Additional studies should focus on factors affecting long term L3 survivability, migrational pattern on these and other plant species and the relationship between climatic factors and larval migration patterns throughout the year. Total larval recovery of H. contortus in this study was greater in bahiagrass than bermudagrass. While the design of this study did not allow for testing one pasture species against another, studies with potted plants would allow for some valid comparisons. Soil characteristics may also play a role in L3 survival and subsequent migration.