Pressure-induced structure ordering of long-chain Fluorescein O322 molecules within the monolayer at the air/water interface was studied by rotating a sample trough using the second-harmonic generation technique. Azimuthal angle dependences of the polarized second-harmonic intensity (I-pp, I-ps, I-ss, and I-sp) were measured for different degrees of monolayer compression. I-pp and I-ss showed a 2-fold pattern, whereas I-ps and I-sp showed a 4-fold pattern. A set of theoretical equations was derived to interpret the observed pattern. The observed patterns agreed with the equations based on C-2v symmetry, and the monolayer showed a definite crystalline structure in the 0.21 nm(2)/molecule region. These results showed that the transition of an initially disordered and heterogeneous dye monolayer into a fairly regular crystalline-like structure was characterized by continuous changes in the structure ordering of constituent molecules maintaining C-2v symmetry.