A novel neurohormone, which anticipates ovarian maturation, was recently purified using liquid chromatography from the African locust nervous corpora cardiaca. Both its function and production by the pars intercerebralis of Locusta migratoria lead to its name, the ovary maturating parsin (Lom OMP). In this study, the Lom OMP was physically and chemically characterized. Its multiply charged ion spectrum was interpreted as two peaks of quite equal size having molecular masses of 6923.4 Da (major peak) and 6907.3 Da. The Lom OMP presented no periodic secondary structure according to the far ultraviolet circular dichroism spectrum obtained. It is composed of 65 amino acids and included a high concentration of alanine but is devoid of cysteine, isoleucine, methionine, lysine and threonine. The amino acid sequence indicated only one microheterogeneity, observed at position 26, consisted in the replacement of serine by alanine. The calculated Mr of the two acidic isoforms (calculated pHi = 4.87) were found to be in agreement with mass spectrometry measurements. When compared to the sequence libraries, the Lom OMP, the first insect gonadotropic neurohormone, was revealed as an unique protein.