The fusion neutron yield during hydrogen neutral beam injection into deuterium plasmas is examined and compared with the neoclassical model, using the Doublet III data obtained in 1983. This provides scalings of thermonuclear fusion neutron yields for the discussion of ion energy transport in tokamak devices. – Experimental data show that fusion neutron yield appears to have a scaling of in low recycling divertor discharges (so called H mode). The neoclassical theory for ion heat conduction satisfactorily described the scaling of fusion neutron yield. With a neutral injection power of Pinj = 4.6 MW, the Doublet III deuterium plasma yields thermonuclear neutrons at a rate of 1.2 × 1013 n·s−1 in low recycling divertor discharge, which is equivalent to a D+ T+ plasma with a thermonuclear fusion power multiplication factor of Q = 7.8 × 10−5 or a beam driven T+ plasma with Q = 8.6 × 10−2. A limiter discharge yields fewer neutrons.