Two experiments have shown that 87Br is the major contributor to the 55-sec delayed-neutron activity in 235U thermal-neutron fission products. An upper limit of 0.5% has been set on the possible contribution to the total 55-sec delayed-neutron activity by unknown precursors with half-life of the order of 55-sec or longer. The half-life of87Br has been accurately measured by observing the neutron decay in chemically pure bromine samples and was found to be 55.4 ± 0.7 sec.