Nonlinear optical measurements show 5‐nitrouracil (5NU) to be phase matchable with a co‐efficient approximately equal to that of lithium iodate [d36(5NU) = (1.0 ± 0.2)d33(LiIO3)]. The large optical nonlinearity is attributed to a highly acentric molecular distribution of non‐&sgr;‐bonded electrons. Air‐stable nonoptically damageable single crystals were grown from an aqueous solution. X‐ray results show the material to crystallize in the orthorhombic space groupP212121containing four molecules per unit cell of dimensionsa0= 9.94 ± 0.03 Å,b0= 10.30 ± 0.03 Å, andc0= 5.47 ± 0.02 Å. The crystals are transparent from 1.5 to 0.5 &mgr;.