The U.S. Bureau of Mines is studying blast delay timing influences on rock fragmentation in a series of tests that started in 3-ft concrete blocks and includes reduced-scale and full-scale bench blasts. This paper reports on the reduced-scale tests. In a 45-in- high dolomite bench, 18 single-row blasts were fired with 15-in burdens. Spacings were 21 and 30 in. Delay intervals ranged from 0 to 45 ms, equivalent to 0 to 36 ms/ft of burden. Each shot was instrumented for strain and pressure for both in situ dynamics and interactions between blastholes. All fragmented rock was screened. The finest fragmentation occurred at blasthole delay intervals of 1 to 17 ms/ft of burden. In this range, stress-wave-induced strains interacted with longer lasting gas-pressure strains from earlier holes. Coarse fragmentation resulted from short delays (<1 ms/ft), where breakage approached presplit conditions with a major fracture between blastholes and large blocks in the burden region. Coarse fragmentation also resulted from long delays (>17 ms/ft), with explosive charges acting independently. The broad acceptable range provides blast design tools useful for a variety of purposes, including optimum muckpile displacement and vibration control.