Single-crystalline CdS nanobelts demonstrate excellent field-emission properties for the first time. Individual CdS nanobelts are assembled for high-performance visible-light photodetectors with an ultrafast response time of similar to 20 mu s and ultrahigh quantum efficiency of 1.9 x 10(7)%. The results suggest a high potential of utilizing these CdS nanobelts as optoelectronic nanodevices.