The response and behavior of postlaval Japanese anchovy (shirasu) to differences in turbidity, prey density, and salinity were studied with the aim of understanding the formation mechanisms of shirasu fishing grounds. An experimental tank was composed of 2 sections divided by a sliding board in which different combinations of turbidity, salinity, and prey density condition were prepared. The behavior of 50 postlarvae released in one section was observed. After removing the sliding board the number of individuals which moved into another section was counted after 30 seconds. In an experiment on various combination of conditions, shirasu showed a strong positive reaction only to turbidity and moved into the section of turbid water. More than 90 percent of shirasu moved from the section without turbidity to the section of turbid water of 5-10ppm, while more than 75 percent moved to the section of 20-30ppm turbidity. At much higher turbidity in the range of 40-50ppm, shirasu were first attracted into the turbid water, but escaped from it at the next moment. Prey density and salinity showed no significant effect in attracting shirasu, although a high prey density may be effective in keeping shirasu in the fishing ground. Turbidity and salinity observations in shirasu fishing grouds in Suruga Bay also support this result.