Length–weight relationships of four pipefish and two seahorse species from Tunisian waters are presented in this paper: the straight-nosed pipefish Nerophis ophidion, the black-striped piperfish Syngnathus abaster, the greater pipefish S. acus, the deep-snouted piperfish S. typhle, the short snouted seahorse Hippocampus hippocampus and the long-snouted seahorse Hippocampus. ramulosus. Overall, 2424 specimens of these six syngnathid species were weighed and measured. The sample size ranged from 14 for N. ophidion to 1773 for H. ramulosus. The values of the slope b in the length–weight relationships (W = aLb) varied between 1.836 (S. typhle) and 5.476 (N. ophidion). Linear regressions of length–weight relationships were significant for all species. We have calculated a positive allometry in weight versus length for N. ophidion, an isometry for H. hippocampus and H. ramulosus and a negative allometry for S. abaster, S. acus and S. typhle.