A study was made of the effects of Na removal on the resting tension of single muscle fibres of the crab Callinectes danae. Reduction of [Na]o (replacement with Li, Tris or choline) below a threshold value, typical for each fibre, induced spontaneous, local contractions randomly dispersed along the fibres; this was followed by propagated contractile waves and tension oscillations. Sustained contractures were occasionally seen at threshold [Na]o and were consistently observed when [Na]o was further reduced. The Na withdrawal contractions depended on [Ca]o and were abolished in Ca‐free media; they were restored within seconds after the addition of Ca (3‐12 mM) or Sr (15‐25 mM), but not Ba (10‐100 mM), to the media. Caffeine (0.2‐1.0 mM) facilitated, whereas La (2‐5 mM), procaine (1 mM) or lidocaine (10 mM) inhibited the Na‐withdrawal contractions. It is concluded that increased Ca influx across the sarcolemma and release of stored Ca from the sarcoplasmic reticulum are involved in the contractions induced by Na‐deficient solutions in crab fibres.