Fission track detectors were applied to a unique problem in neutron dosimetry. Measurements of neutron doses were required at locations within a tumor of 1 cm diameter implanted on the back of a mouse and surrounded by a square array of four252Cf medical sources. Measurements made in a tissue‐equivalent mouse phantom showed that the neutron dose rate to the center of the tumor was 2.18 rads μ g−1h−1±8.4%. The spatial variation of neutron dose to the tumor ranged from 1.88 to 2.55 rads μ g−1h−1. These measurements agree with calculated values of neutron dose to those locations in the phantom. Fission track detectors have been found to be a reliable tool for neutron dosimetry for geometries in which one wishes to know neutron dose values which may vary considerably over distances of 1 cm or less.