[en] The altitude distributions of cosmic-ray-produced neutron fluxes were measured in the lower atmosphere near the air - ground boundary at a geomagnetic latitude of 25⁰N. A pair of large BP-28 BF₃ proportional counters, each enclosed in a 2-cm polyethylene tube, were hung down out of a helicopter. One counter was covered by a cadmium sheet to separate the contributions from thermal and fast neutrons. Flights were carried out over land at two different altitudes and over seawater at various levels up to 1000 m above the surface. At high altitudes the attenuation length is 145 g/cm²; this indicates an equilibrium condition for the neutrons at this latitude. Abrupt reductions of the attenuation length of fast neutrons exist at the same height. Further altitude variations were obtained by a mobile survey along a mountain road. The counting rates vary in parallel with those in free air, being higher in free air for fast neutrons and higher on land for thermal neutrons. These results can be interpreted in terms of the air - ground boundary effects on the cosmic-ray neutron-flux distribution