We use contemporaneous causality tests based on instrumental variables (IV) methods to re-examine causality from Commodity Index Trader positions to agricultural futures prices. A number of recent studies found that no Granger-causal impacts are discernible for agricultural commodity markets. Market microstructure theory suggests that the impact of index-based trading on agricultural futures markets should be contemporaneous and the data should fail to support Granger-causality despite the presence of a contemporaneous causal. IV-based tests for contemporaneous causality provide evidence for impacts in six of the twelve commodities. These are predominantly the less active contracts as measured by open interest.