I presented evidence that sperm transfer can occur during forced copulation (FC) under natural conditions, and tested predictions of the fertilization strategy hypothesis, with field data from a marked population of lesser scaup breeding in southwestern Manitoba. As predicted, males directed FC attempts at fertilizable females and seemingly had tactics that increased their effectiveness in inseminating such females. Males selected older females (the more productive) when attempting FCs in 1 year of the study. FC rates were highest in the morning when chances of successful fertilization were probably highest. FC rates were positively correlated with annual variations in female reproductive effort and performance, but the exact nature of the relationship requires further study. Mate-and nonmate-defense by males and other behaviours which probably function as anticuckoldry tactics were observed. Contrary to BARASH'S (1977a) prediction, paired males attempted FCs more frequently than did unpaired males. Although pair formation through mate-switches is probably a more profitable strategy than FC for unpaired males, FC may be an important "last resort" strategy for older, experienced unpaired males late in the season. In general, my results provided strong support for the male fertilization strategy hypothesis.