Extracellular Ca 2+ mediates the cellular and molecular responses to cell stimulation ofChlamydomonas reinhardtii.Extracellular Ca 2+ concentrations ([Ca 2+ ] e ) must exceed certain threshold values to support flagellar excision by acid shock and to stimulate flagellar outgrowth following mechanical shear of the flagella. Also, the magnitude and duration of flagellar RNA accumulations following acid shock or mechanical shear increase with increasing [Ca 2+ ] e . To better understand the role of Ca 2+ in flagellar excision, RNA induction, and outgrowth, we have performed a survey of the ion selectivity of each of these responses to acid shock. We found that flagellar excisionin vivofollowing acid shock was supported by Sr 2+ and Ca 2+ , but no other ion tested. LaCl 3 and neomycin prevented flagellar excision following acid shock of cells in Ca 2+ - or Sr 2+ -containing buffer. Sr 2+ addition to detergent-permeabilized cell models, however, failed to elicit flagellar excisionin vitro.Cells failed to regrow flagella following flagellar excision in Sr 2+ -containing buffer unless exogenous Ca 2+ was added. Flagellar RNA accumulations of lower magnitude and shorter duration were measured in cells acid-shocked in Sr 2+ -containing buffer than in Ca 2+ -containing buffer. These results demonstrate that a Sr 2+ influx can evoke flagellar excision following acid shock, but cannot directly activate the machinery for flagellar excision, suggesting that a Sr 2+ influx induces excision by stimulating an intracellular Ca 2+ release. Furthermore, they suggest that flagellar outgrowth and normal flagellar RNA induction have a strict requirement for Ca 2+ , which is not satisfied by the proposed intracellular Ca 2+ release.