Is this transducer xhs 9 hdsi 180 t compatible with my Humminbird 999 ci HD?

Yes, the Humminbird XHS-9-HDSI-180T TM Transducer is compatible with the Humminbird 999ci HD fishfinder. This transducer is specifically designed to work with Humminbird units that support Side Imaging, Down Imaging, and DualBeam PLUS sonar technologies, which are features present in the Humminbird 999ci HD.