In this paper we present length-weight relationships of three pipefishes, Syngnathus acus, Syngnathus typhle Nerophis ophidion, and two seahorse species, Hippocampus hippocampus, Hippocampus guttulatus from izmir Bay, on the Turkish coasts of the Aegean Sea. Overall, 1010 specimens of five different species of Syngnathidae were weighed and measured. The sample size ranged from 29 for H. hippocampus to 570 for S. acus. The values of the exponent b in the length-weight regressions (W=aLb) varied between 2.42 (N. ophidion) and 3.54 (S. acus). Linear regressions of length-weight relationships were significant for all species. Positive allometry in weight vs. length for S. acus, isometry in S. typhle and H. hippocampus and negative allometry for H. guttulatus and N. ophidion were observed.