The rate of nucleation of crystalline dimethylacetylene in the neat supercooled liquid has been measured in 130 Angstrom clusters containing similar to 11 000 molecules, Clusters were generated by condensing the vapor in supersonic flow with neon carrier. At the rate of cooling to which they were subjected, the clusters dropped far below the bulk melting point of 241 K before they began to freeze at similar to 155 K into a heretofore unknown solid phase, This solid had a density of packing close to that of the known tetragonal phase at a similar temperature. The maximum freezing rate was attained at 151 K where the nucleation rate reached 1.14 x 10(29) critical nuclei m(-3) s(-1). Clusters froze to single crystals, consistent with Kashchiev’s criterion for mononuclear freezing when it was applied with estimated rates of growth of postcritical nuclei. Because no thermodynamic information about the new solid phase was available, it was not possible to analyze the kinetic information in terms of the classical theory of homogeneous nucleation without introducing assumptions. However, plausible estimations of unknown quantities led to an interfacial free energy of the solid-liquid boundary of similar to 20.0 mJ/m(2), a value in rough conformity with Turnbull’s empirical rule. Differential scanning calorimetry carried out on a sample frozen by plunging the fluid into liquid nitrogen showed no signs of a transition at the normal tetragonal-to-tetragonal transition temperature. Whether the signal seen 41 deg higher involves the new solid phase produced in supersonic flow is not yet known.