Langmuir-Blodgett (LB) films of manganese octadecylphosphonate (Mn(O3PC18H37). H2O) are prepared by depositing octadecylphosphonate monolayers from a MnCl2 . 4H(2)O containing subphase at a pH range of 5.2-5.6. Electron paramagnetic resonance (EPR) studies of the LB film give evidence for antiferromagnetic exchange in a two-dimensional inorganic extended lattice, in agreement with published magnetic studies of solid-state manganese alkylphosphonates. The g values of the LB film are characteristic of Mn2+ in a nearly cubic field and are essentially isotropic, ranging from 1.99 to 2.00. The dependence of the EPR line width on sample orientation in the magnetic field is consistent with the behavior predicted for a two-dimensional lattice with antiferromagnetic Heisenberg exchange. The temperature dependence of the integrated area of the EPR signal, which is proportional to spin susceptibility, is presented. A value for the antiferromagnetic exchange constant, J/k, of -2.8 K was determined for the LB film by fitting the data to a high temperature series expansion. This is in excellent agreement with the J/k values obtained for the solid-state manganese phosphonates. EPR line widths of both the manganese octadecylphosphonate LB film and a powder sample of manganese propylphosphonate increase rapidly as temperature is decreased below 30 K. This is characteristic of a system approaching a magnetic ordering transition and is caused by antiferromagnetic fluctuations. The solid-state manganese alkylphosphonates undergo antiferromagnetic ordering transitions at temperatures in the range 14.8-15.1 K, but an ordering transition in the LB film cannot be observed using EPR because the line width becomes too broad and the signal is too weak to measure below 17 K. The study confirms that the in-plane structure of the manganese phosphonate LB film is the same as that of the solid-state manganese phosphonate analogs.