This repository provides the data, code, and documentation associated with the study: 'A sensorimotor instability drives a locomotor transition during fish development' The material documents the full experimental and computational pipeline used to study swimming behavior and locomotor transitions during development in freely swimming Danionella fish. It includes: Code and documentation for the experimental tracking system, including hardware configuration, real-time object detection, and video acquisition Post-processing pipelines for posture segmentation and trajectory reconstruction Processed datasets aggregated across individual fish and developmental stages Analysis code and notebooks reproducing all main and supplementary figures of the paper Computational fluid dynamics (CFD) simulation data and analysis used to quantify swimming efficiency across body sizes and swimming modes The repository is organized in a modular manner to facilitate selective access, allowing users to download only the components relevant to their interests. Due to size constraints, only representative example of the raw datasets are included; full datasets are available in the corresponding analysis modules. This archive represents a citable snapshot associated with the published study and is not intended as a continuously maintained software package. Code is provided to ensure transparency and reproducibility of the reported results.