This paper draws regulatory lessons for emerging stock markets from an empirical study of the relationship between transactions costs and share price volatility in the London Stock Exchange. We concentrate our analysis on direct pecuniary costs of trading, namely transactions taxes (stamp duty) and brokerage charges, which derive directly from regulation. In a novel contribution to the transactions cost literature, we identify stock market performance with various measures of market volatility, and distinguish among market volatility, fundamental volatility and excess volatility; we also propose some simple ways of identifying the separate impact of transactions costs on these volatility measures. Our findings suggest that changes in transactions costs have a significant and dependable effect on share price volatility but the sign of this effect depends critically on the concept of volatility being measured. Among the important lessons for emerging stock markets is that transactions costs are an important factor in share market volatility and the regulatory regime therefore needs to take account of the impact of regulation on such costs. This is particularly important for those emerging stock markets that rely on stamp duty or other transactions taxes as a regulatory tool.