Our objective was to assess within-herd seroprevalence of Anaplasma marginale antibodies across 12 Florida beef cattle herds and compare with statewide seroprevalence. Twelve surveyed herds ranged in size from 160 to 456 adult Bos taurus – Bos indicus cattle. Statewide A. marginale seroprevalence relied on blood samples obtained at slaughter from 201 Florida cattle. Screening relied on competitive ELISA. Before serology, an outbreak of anaplasmosis resulted in increased mortality (up to 17.8%) and abortions in several herds. Up to 29.2% of cows aborted late in gestation in 2 herds that included many cattle introduced from Texas. Among 1,085 cattle tested in the 12 herds, seroprevalence of A. marginale varied from 2.6 to 85%, with an overall seropositive rate of 50.3%. Cattle in open herds were 6.23 (95% CI: 4.26–9.17) times more likely to experience mortality and 3.10 (95% CI: 2.39–3.98) times more likely to abort than animals in closed herds. Average mortality (12%) and abortion (16.3%) among open herds were significantly ( P < 0.05) higher than mortality (1.9%) and abortion (5.3%) among closed herds. Overall seropositivity among affected herds was higher than the apparent statewide seroprevalence of 20.32%. This survey provides estimates of seropositivity among Florida cattle and reports the absence of uniform herd immunity in an area considered endemic for bovine anaplasmosis. These data highlight unrestricted cattle movement and environmental conditions that favor vector-borne disease transmission as risk factors for disease outbreaks even in regions that are considered endemic for bovine anaplasmosis.