Association between albendazole anthelmintic resistance and a panel of life history traits in Heligmosomoides polygyrus was investigated both prior to and during drug-selection. Associations prior to anthelmintic application were studied in ten independent lines isolated without drug treatment from a susceptible stock population by random genetic drift. Variation among lines was detected in several life history traits (i.e., establishment, development and survival), and, despite lack of previous exposure to albendazole, lines also varied in their tolerance to the drug. No significant correlations were detected between drug-tolerance and any of the life history traits after 11 generations of isolation. The apparent lack of fitness differential between lesser and more drug-tolerant individuals of the susceptible population is not in accordance with the assumption that the low frequency of drug tolerant individuals in the susceptible population is explained by their lower fitness, but is consistent with the neutral theory. Associations between life history traits and drug resistance were also studied using two lines selected for albendazole resistance from the stock population, and two control lines exposed to the same monthly passage procedure but not to the drug. After 10 generations of selection, drug resistance increased from an LC50 of 0.48 $ mu$M to 2.03 $ mu$M. In a primary infection, the higher establishment and higher worm numbers one-month post-infection in the resistant parasites compared to the stock parasites occurred only in the drug-selected lines. Changes in these traits were attributed to the drug selection regime. In contrast, both drug-selected lines and passaged lines showed a faster rate of development and higher early egg production compared with the stock parasites; these changes were attributed to the passage procedure that presumably acted as a selective force on early life history traits. In immunized hosts, changes in several traits that o

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