Brazil has the largest stock market in South America; Argentina has one of the smallest. We investigate the spread relationship between these two markets, measured as the ratio of Brazil s Bovespa index to Argentina s Merval index. Using rescaled range analysis, we identify the presence of a time-varying fractal structure in this ratio. When a Hurst-based trading rule is applied, we find that episodes of fractality may be exploited by traders. Under some circumstances, these strategies are more profitable than economic gains from simple moving average systems, which exploit the autocorrelation structure of the series.