In the last decade the number of papers related to wireless sensor networks has increased substantially. Most of them focus in theoretic issues such as routing algorithms, network lifetime and so on. In contrast with all those studies, we present a practical application of wireless networks: an online water monitoring system based on ZigBee and GPRS. The contents of several chemicals in the water are sensed. The sensor data are collected and transmitted via ZigBee and GPRS. Special techniques are involved in the implementation of the system while employing TinyOS, LabVIEW and MySQL.