Abstract. The relatively low magnetic fields used in spherical torus (ST) plasmas means that energetic ions can have a large Larmor radius, of the same order of magnitude as the scale length for changes in B. The magnetic moment, , is not well conserved in this limit and the fast ions can diffuse in pitch angle as a result. The magnitude of this diffusion is being calculated theoretically, and measurements of the neutron rate produced by beam ions in NSTX are being analyzed to look for evidence of this transport. Preliminary measurements place an upper bound of no more than 20% loss fraction attributable to mu non-conservation. The effect is limited in NSTX due to the fact that the beam injection is purely in t he co-direction, and it may be more severe for alpha particles in a DT-fueled ST, whose population would be half counter-going. Other measurements indicate that MHD activity in NSTX can have a very strong effect on the beam ion population in the plasma.