In order to obtain the basic information for seeding production of Echiuroid worm, Urechis unicinctus, the influence of pH and salinity on egg development was investigated. Mature adult of U. unicinctus were collected at the Diving Cooperation of Yosu in Korea and reared during 5 weeks. We carried out the artificial insemination in the laboratory on Dec. 29, 1998, and reared the embryo under different pH and salinity. Treatments were carried out with different pH(4~10) and salinity($0~45 extperthousand$). Embryos in pH 4, salinity $0 extperthousand$, $10 extperthousand$, $40 extperthousand$ and $45 extperthousand$ tanks did not develope after fertilization and became deformed or dead, before swimming embryo. In these pH and salinity conditions, deformation rate of embryo was high at 8-cell stage and 16-cell stage. But embryos in pH 5~10, salinity $20~35 extperthousand$ tanks developed into swimming embryo stage. These result indicate that an echiuran inhabits in both intertidal and subtidal mudflates. After fertilization, sixteen-cell stage took 5.3~5.6 hours in pH 5~10 tanks, and 5.1~5.8 hours in $20~35 extperthousand$ tanks. And swimming embryo took 13.3~ 14.1 hours in all conditions. The desirable pH and salinity for egg development were 7~8 and $30 extperthousand$, respectively.