We examine the short-run relationship between stock-return volatility and daily equity trading by several investor groups in the Korean Stock Exchange. We also investigate whether trade characteristics and trading styles can explain the potential distinct volatility effects of these investor groups. For large stocks, we find that whether a trade is a purchase or a sale and whether it is a contrarian or a momentum trade does not play a role in the relation between volatility and trading. It is the trading of informed institutional investors against non-informed individual investors that drives volatility and produces a negative volatility effect. We further show that net foreign trading has a non-decreasing impact on volatility. Our results are robust to alternative measures of volatility and obtained after controlling for a Monday effect, volatility persistency, total volume and lagged stock returns.