David J. Garbary, Gayle I. Hansen and Robert F. Scagel: The marine algae of British Columbia and Northern Washington: Division Rhodophyta (Red Algae), Class Florideophyceae, Orders Acrochaetiales and Nemaliales This is a floristic treatment of red algae (Division Rhodophyta) in two orders (Acrochaetiales and Nemaliales) of the Class Florideophyceae occurring in British Columbia and northern Washington. It provides keys, descriptions, and illustrations for the 31 species in six genera of these orders as they are known to occur in the area. This includes 25 species in the only genus (Audouinella) in the Acrochaetiales and six species in the genera Bonnemaisonia, Pseudogloiophloea, Whidbeyella, Cumagloia and Nemalion in the Nemaliales. Nine new combinations are proposed in Audouinella. Since this paper is the first in a series on the marine Florideophyceae of British Columbia and northern Washington, a general summary and introduction to the class is presented, together with keys to all orders in the class. One of these keys is based on criteria relating to the female reproductive system and postfertilization developments and the other is a key based primarily on vegetative features of marine Florideophyceae in British Columbia and northern Washington.