We studied by small angle x-ray scattering (SAXS) the tetramethylurea (TMU) effects on the tertiary structure of lysozyme in water. Samples containing TMU with mass fractions, w, ranging from 0 to 0.7 were analyzed. The SAXS results clearly demonstrate a structural transition from a spheroidal molecule (0 less than or equal to w less than or equal to 0.6) to a highly anisometric molecule for w = 0.7. Additional SAXS results for samples with w = 0.7, from which the TMU was removed, showed that this transition is reversible. The variation of the maximum dimension of the molecule with w fits an equation corresponding to a ＇＇two-state＇＇ model, At w = 0.6 (critical fraction) a mixture of folded (spheroidal) and unfolded (anisometric) molecules coexists in the solution.