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The objective of this study was to observe for an association between flooring surface and the onset and expression of estrus as measured by radiotelemetry. Twenty-four primiparous Holstein cows in early lactation were housed in an open, free-stall barn with concrete flooring. Cows were moved to a dirt pen adjacent to the free-stall barn for 3-h periods preceding AM and PM milkings (06:00 and 18:00). Each cow received a radiotelemetric transmitter to record mounting activity. Data were analyzed using chi-square, where the expected onset of estrus occurred randomly throughout the day. The total number of mounts recorded was 147. Eighty-two mounts (55.8%) were recorded during the two time periods of 3 h each, from 03:00 to 06:00 h and from 15:00 to 18:00 h, coinciding with the time when cows were in the dirt pen (2 = 74.3; p < 0.0005, 7df) compared to 65 mounts (44.2%) recorded during the remaining 18 h when cows were in the free-stall barn. Seventeen cows (70.8%) received the first mount in the dirt pen (2 = 26.9; p < 0.0005, 7df) and within that measure, seven cows (29.2%) received mounts only in the dirt pen. The total number of estrous bouts was 31. The onset of twenty-one estrous bouts (67.8%) occurred when cows were in the dirt pen (2 = 31.8; p < 0.0005, 7df) and within that measure, standing activity for eleven bouts (35.5%) was expressed only in the dirt pen. The mean duration of estrus was 5.2 0.9 h. The average number of mounts during estrus was 4.7 0.9 and the mean duration of mounts was 3.2 0.3 s for a total mean mounting duration per estrus of 15.3 2.7 s. Results of the present study indicate that flooring surface is associated with the onset and expression of estrus.