From low-temperature heat capacity and ac susceptibility data it is inferred that YbNiSb orders magnetically at 0.8 K. An unusually large magnetic-field dependence of the heat capacity measured up to 20 K, and also a large decrease in the electrical resistivity ([similar to]1200 [mu][Omega] cm) below 150 K are observed. These data suggest an appreciable dynamical mass renormalization of the quasiparticles due to a 4[ital f] conduction-band mixing effect.