A novel caldoactive filamentous bacterium was isolated from a New Zealand hot spring. Optimum conditions for growth were pH 7.0 and 73°C in aerobic conditions. Growth also occurred anaerobically. Acetamide, lactose, nitrilotriacetic acid, succinate and starch could all act as sole carbon source. Carotenoids could be extracted and had a peak absorbance at 450 nm. The organism was Gram-negative. Long chains of cells were formed and showed characteristic coiling, necridia, and bulbs. The organism is compared to other caldoactive filamentous organisms and is sufficiently different to make it probable that it represents a new taxonomic group capable of life at high temperatures.