<jats:title>ABSTRACT</jats:title> <jats:p> The bacterial strain MM-B16, which showed strong antifungal and antioomycete activity against some plant pathogens, was isolated from a mountain forest soil in Korea. Based on the physiological and biochemical characteristics and 16S ribosomal DNA sequence analysis, the bacterial strain MM-B16 was identical to <jats:italic>Pseudomonas fluorescens</jats:italic> . An antibiotic active against <jats:italic>Colletotrichum orbiculare</jats:italic> and <jats:italic>Phytophthora capsici</jats:italic> in vitro and in vivo was isolated from the culture filtrates of <jats:italic>P. fluorescens</jats:italic> strain MM-B16 using various chromatographic procedures. The molecular formula of the antibiotic was deduced to be C <jats:sub>10</jats:sub> H <jats:sub>11</jats:sub> NO <jats:sub>2</jats:sub> S (M <jats:sup>+</jats:sup> , <jats:italic>m/z</jats:italic> 209.0513) by analysis of electron impact mass spectral data. Based on the nuclear magnetic resonance and infrared spectral data, the antibiotic was confirmed to have the structure of a thiazoline derivative, aerugine [4-hydroxymethyl-2-(2-hydroxyphenyl)-2-thiazoline]. <jats:italic>C. orbiculare</jats:italic> , <jats:italic>P. capsici</jats:italic> , and <jats:italic>Pythium ultimum</jats:italic> were most sensitive to aerugine (MICs for these organisms were approximately 10 μg ml <jats:sup>−1</jats:sup> ). However, no antimicrobial activity was found against yeasts and bacteria even at concentrations of more than 100 μg ml <jats:sup>−1</jats:sup> . Treatment with aerugine exhibited a significantly high protective activity against development of phytophthora disease on pepper and anthracnose on cucumber. However, the control efficacy of aerugine against the diseases was in general somewhat less than that of the commercial fungicides metalaxyl and chlorothalonil. This is the first study to isolate aerugine from <jats:italic>P. fluorescens</jats:italic> and demonstrate its in vitro and in vivo antifungal and antioomycete activities against <jats:italic>C. orbiculare</jats:italic> and <jats:italic>P. capsici</jats:italic> .