Electrical and mechanical power, together with other forms of useful work, are generated worldwide at a rate of about 10(12) watts, mostly using heat engines. The efficiency of such engines is limited by the laws of thermodynamics and by practical considerations such as the cost of building and operating them. Engines with high efficiency help to conserve fossil fuels and other natural resources, reducing global-warming emissions and pollutants. In practice, the highest efficiencies are obtained only in the most expensive, sophisticated engines, such as the turbines in central utility electrical plants. Here we demonstrate an inexpensive thermoacoustic engine that employs the inherently efficient Stirling cycle(1). The design is based on a simple acoustic apparatus with no moving parts. Our first small laboratory prototype, constructed using inexpensive hardware (steel pipes), achieves an efficiency of 0.30, which exceeds the values of 0.10-0.25 attained in other heat engines(5,6) with no moving parts. Moreover, the efficiency of our prototype is comparable to that of the common internal combustion engine(2) (0.25-0.40) and piston-driven Stirling engines(3,4) 4 (0.20-0.38).