Methods are provided for synthesizing ZSM-58 crystals with an improved morphology and/or an improved size distribution. By controlling the conditions during synthesis of the ZSM-58 crystals, crystals of a useful size with a narrow size distribution can be generated. Additionally, by controlling the ratio of water content to silica content in the synthesis mixture, it has unexpectedly been found that ZSM-58 crystals can be formed with an improved morphology. The improved morphology can result in ZSM-58 crystals with a more uniform size across the various dimensions of the crystal, which allows for more uniform diffusion within the crystal. This is in contrast to conventionally synthesized crystals, where the size of the crystal can vary along different axes of the crystals.