The magnetic properties of the cubic Laves phases (C-15 structure) NpMn2, NpFe2, NpCo2, and NpNi2 have been studied from 4 to 300 K by means of magnetization, neutron-diffraction, and nuclear-γ-ray resonance (Mössbauer) experiments. The respective ordering temperatures (and types of ordering) are 18 K (ferro), ~500 K (ferro), 15 K (antiferro), and 32 K (ferro). Magnetic moments are present on the neptunium atoms in all compounds with values varying from 1.2μB in NpNi2 to 0.3μB in NpMn2. On the transition-metal site, only the iron atom carries a magnetic moment in zero field; in high magnetic fields, a small moment of ~0.2μB is observed on the manganese atom. No simple correlation appears to exist among the values of the magnetic moments, the Np-Np distance, and the ordering temperature. A feature of the data is the large high-field susceptibility in the ferromagnetic compounds at low temperatures. Some magnetic properties, such as the large anisotropy (<111> is the easy axis) and the linear relationship between the hyperfine field and the neptunium magnetic moment, are best interpreted in terms of localized 5f electrons on the actinide ion, whereas other properties, such as the low values of the neptunium moments and the high-field susceptibility, suggest strong hybridization between the 5f and 3d electrons.