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Abstract Sphingolipids have roles as membrane structural components and as bioactive molecules in plants. InPhyscomitrella patens, 4‐hydroxysphinganine (phytosphingosine, t18:0) is the predominant sphingolipid long‐chain base (LCB). To assess the functional significance of t18:0, CRISPR‐Cas9 mutagenesis was used to generate mutant lines lacking the soleSPHINGOID BASE HYDROXYLASE(SBH) gene encoding the hydroxylase responsible for converting sphinganine (d18:0) to t18:0. Total sphingolipid content insbhprotonemata was 2.4‐fold higher than in wild‐type. Modest changes in glycosyl inositolphosphorylceramide (GIPC) glycosylation patterns occurred. Sphingolipidomic analyses of mutants lacking t18:0 indicated modest alterations in acyl‐chain pairing with d18:0 in GIPCs and ceramides, but dramatic alterations in acyl‐chain pairing in glucosylceramides, in which 4,8‐sphingadienine (d18:2) was the principal LCB. A striking accumulation of free and phosphorylated LCBs accompanied loss of the hydroxylase. Thesbhlines exhibited altered morphology, including smaller chloronemal cell size, irregular cell shape, reduced gametophore size, and increased pigmentation. In the presence of the synthetic trihydroxy LCB t17:0, the endogenous sphingolipid content ofsbhlines decreased to wild‐type levels, and the mutants exhibited phenotypes more similar to wild‐type plants. These results demonstrate the importance of sphingolipid content and composition to Physcomitrella growth. They also illuminate similarities in regulating sphingolipid content but differences in regulating sphingolipid species composition between the bryophyteP. patensand angiospermA. thaliana.