Abstract: Brown spot needle blight is one of important pine diseases in South China.The pathogen fungus ( Lecanosticta acicola )could produce toxic substances to make the needle wilt.Rp 18 reversed phase silica gel column was repeatedly used to separate crude LA toxins. One colorless needle crystal was obtained.It is a phytotoxic substance named LA Ⅱ.HPLC analysis indicated LA Ⅱ was a pure compound. The molecular weight and molecular formula were determined by FAB MS and HRFAB MS as 118 and C 4H 6O 2S respectively. According to IR, MS, NMR spectra data, the structure of LA Ⅱ was elucidated as CH 3COSCOCH 3.