Seasonal dynamics of helminth parasites affecting indigenous ducks and the effects of parasites on Production performance under semi scavenging conditions were studied. To study the seasonal dynamics ducks were collected from different areas of Mymensingh district and subjected to routine Post-mortem examination for the collection of helminth parasites. A total of 300 ducks were examined of which 295 (98.33%) were positive for one/more species of helminth parasites. Thirty-one species of helminth parasites were identified, among them 16 species belonged to .trematodes, 8 species belonged to cestodes, 5 species belonged to nematodes, and 2 species belonged to acanthocephala. Relatively higher prevalence was recorded in rainy season (100%) followed by summer (98.10%) and winter (97.99%). In case of trematodes and acanthocephala,.the infection rate was the highest in rainy season (100% and 28.26%) followed by winter (89.93% and 7.38%) and summer (83.80% and 4.76% respectively). But in case of cestodes and nematodes first peak was observed in rainy season (67.39% and 47.83%) followed by summer (59.80% and 20.95%) and winter (42.95% and 17.45% respectively). The mean density of trematodes (21.93±2.05), cestodes (79.84±4.19), nematodes (6.82±1.13) and acanthocephala (52.08±2.59) was also the highest during rainy season. However, these variations were not statistically significant. On the other hand, this study revealed that the helminth parasitic infection greatly hampered the body weight gains and egg production. The mean body weight in the anthelmintic treated group was 1496.00±20.80 gm but in the untreated control group it was 1182.00±34.17 gm which was significant at 1% level. Besides, the onset of egg production was 32 days earlier in the treated group. The ducks of the treated group laid more eggs, 9.75 eggs /bird in contrast to 5.55 eggs/ bird in control group. The highest egg production was 55% in the treated group while in the untreated group it was 30%. The mean egg production per month in the treated group was 54.33±34.51 and in untreated group 37.00±31.19, which was significant at 5% level Moreover the birds of the treated group also laid relatively larger eggs. The mean weight of eggs of the treated group was 63.24±1.44 gm but that of untreated group was 55.60±1,50 gm.