INTRODUCTION.............................................................. 419 Semantic Problems ............................................. 419 Occurrence of Versatile and Specialist Phototrophs, Lithotrophs, and Methylotrophs ...................................................... 421 CHARACTERISTIC ENERGY AND CARBON METABOLISM ....... ............ 423 Energy Metabolism ...................................................... 423 Carbon Metabolism ...................................................... 423 METABOLISM OF ORGANIC COMPOUNDS BY SPECIALIST PHOTOTROPHS, LITHOTROPHS, AND METHYLOTROPHS ................................. 424 Influence of Organic Compounds on Growth ........... ....................... 424 Assimilation of Organic Compounds ................. ......................... 425 Requirements for the Assimilation of Organic Compounds ....... .............. 427 Fate of Organic Carbon Assimilated by Specialist Strains ....... ............... 427 Substrate Transport and the Specialist Condition............................... 432 OTHER POSSIBLE CAUSES OF SPECIALIST CHARACTERISTICS ...... ...... 433 Regulation of Enzyme Synthesis and Activity .......... ....................... 433 Energy Conservation in Specialist Strains ............. ........................ 434 Specialized Nutritional Requirements ................ ......................... 437 CONCLUSIONS ........................................................... 437 LITERATURE CITED ......................................................... 440