Assemblages of agglutinated foraminifera from the Carboniferous (late Westphalian to Stephanian) coal-bearing strata of the Sydney basin, Cape Breton Island (documented for the first time in 150 yr of investigation of the Nova Scotian coal basins) indicate that deposition took place on an extensive coastal platform, not in a fluviolacustrine setting as previously suggested. The assemblages contain representatives of genera that are widely distributed in modern estuarine and marsh environments, allowing us to determine Carboniferous paleoenvironments with unusual confidence. Seat earths beneath several coal seams contain assemblages dominated by Trochammina, which suggests deposition within vegetated zones analogous to modern high-marsh environments found within the upper half of the tidal range. A coal-seam split and a coarse siltstone contain assemblages dominated by Ammobaculites and Ammotium, which indicate paleoenvironments analogous to modern low-marsh and estuarine environments. Mixed assemblages of Trochammina, Ammobaculites, and Ammotium also indicate estuarine-low-marsh depositional environments. Detailed analysis of foraminiferal biofacies assists in the recognition of marine influence during deposition of these Carboniferous rocks, and may help distinguish areas of sulfur-rich coal. The findings indicate that environmental and morphological conservatism has characterized agglutinated marsh foraminifera for the past 300 m.y.