HD 4539 is one of the brightest known members of the class of early-type high-latitude stars with very weak helinm lines Its spectrum is compared to that of the standard main-sequence star L Her (which has already been studied by others in detail), and a differential model-atmosphere abundance analysis is reported. The He/H ratio is found to be 16 times smaller in the subdwarf than it is in normal B stars of the solar neighborhood. The abundance peculiarities of this star are not confined to helium. The N/H ratio is higher than normal by a factor of 3 and the 0/H ratio is lower than normal by a factor exceeding 10 while the C/H ratio is normal. `i,T speculate concerning the origins of this remarkable composition, but have no convincing hypothesis to put forward. Ve argue that the helium deficiency in this star cannot be regarded as evidence for a low primordial helium abundance.