Organic cows (n = 57) were used to evaluate activity, rumination time and their correlation with pasture fly activity of cows fed 3 grain supplementation strategies during the grazing season from May to September 2013. Cows were assigned to 1 of 3 replicate supplementation groups: 1) no corn grain supplementation (100% pasture, GRASS, n = 19), 2) low corn grain (2.72 kg/head/day, LO, n = 19), and 3) high corn grain (5.44 kg/head/day, HI, n = 19), and calved during 2 seasons (fall and spring) at the University of Minnesota West Central Research and Outreach Center, Morris, from October to December 2012 and March to May 2013. Supplement (organic corn grain and minerals) was fed with a TMR of corn silage and alfalfa silage, and at least 30% of diet DMI for LOW and HI cows consisted of organic pasture. Activity and rumination time (daily and 2-h periods) were monitored electronically using HR-LD Tags (SCR Engineers Ltd., Netanya, Israel) for 125 days. Activity is reported in “activity units” from SCR DataFlow II software. The PROC HPMIXED of SAS was used for statistical analysis, and independent variables were season of calving (fall or spring), month of grazing (June to September), parity (1, 2, 3+), supplementation group and the interactions of month of grazing and supplementation group, and parity and supplementation group. Cow and replicate were random effects with repeated measures. The GRASS (1,138) cows had greater daily activity than HI (1,001) cows, but were similar to LO (1,019) cows. Daily activity was greatest during July (1,258) and least during September (819). Rumination was not different for the GRASS (397 min/d), LOW (384 min/d), and HI (370 min/d) cows. Daily rumination was greater during September (402 min/d) compared to July (361 min/d). Daily activity increased rapidly from h 6:00 and 8:00 to h 16:00 and 18:00. From h 18:00 to 20:00, cows had a rapid decline in activity until h 6:00 the next day. All supplementation groups had the greatest rumination during h 2:00 and 4:00 and the least during h 10:00 and 12:00. Greater activity of cows on a herd basis was moderately correlated with increased pasture fly prevalence. Monthly activity and rumination patterns of grazing organic cows may have been influenced by the weather and fly populations.