Abstract:As an alian species Bursaphelenchus xylophilus replaced B.mucronatus gradually.Prelimemary study suggested their fecundity difference was the reason behind.The fecundity of their offspring under competitive and noncompetitive condition was compared.B.xylophilus showed stronger fecundity than B.mucronatus.The mixture of B.xylophilus and B.mucronatus was cultured for a long time,B.xylophilus replaced B.mucronatus gradully.The fecundity of B.xylophilus under competitive condition was stronger than under noncompetitive condition.