Specimens of the articulated coralline alga Calliarthron tuberculosum growing on an underwater reef off the coast of central California were photographed over a period of 2 years, during which time the mean increase in branch length was 1.7 mm per month. A tier of medullary cells 50–75 μ high is formed about once a day and flexible genicula develop every 1.5–2.7 months. Mean growth rates are 1.3 mm per month in summer (April–Sept.) and 2.3 mm per month in winter (Oct–March). On glass and plexiglass settling surfaces placed among the algae, basal crusts under 1 mm in diameter were visible in less than 1 month, fronds were evident after 2 months, and the fronds bore conceptacles after 11 months. Measurements of photographed and collected specimens reveal that the plants mostly branch during two different times of the year, from March to June and again from September to December.