Two major β-glucan binding proteins (β-GBPs), 75.9 ku and 83.2 ku, were extracted from the hemolymph of Plutella xylostella larvae by using β-1,3-glucan affinity precipitation method. The proteins were present in plasma but absent in haemocytes. The β-GBPs had a specific affinity to β-1,3-glucan, a prophenoloxidase (ProPO) trigger in P.xylostella hemolymph. The phenoloxidase (PO) activity induced by the coexistence of both β-GBPs and β-1,3-glucan was significantly higher than that by the β-GBPs or β-1,3-glucan alone. Moreover, the PO activities induced by mycelial lysates of four Zoophthora radicans isolates at the presence of β-GBPs were much higher than that triggered by their protoplast lysates. The results indicate that the β-GBPs may trigger the ProPO activity in P.xylostella hemolymph only if they specifically recognized β-1,3-glucan from Z.radicans cell walls. This is a proof for a hypothesis that entomophthoralean protoplasts may evade from insect host immune defense. The protoplast or mycelial lysates of different Z.radicans isolates differed in ability to trigger the PO activities in the following order: ARSEF1342＞ARSEF2699＞F99101＞ARSEF1100. This trend was in accordance with that of their virulence to P.xylostella larvae, suggesting a possible relationship between host immune defense and fungal virulence.