In this paper, we examine variations in the critical frequency foF2 recorded by an ionosonde, and the total electron content (TEC) derived from a network of 5 ground-based receivers of the global positioning system (GPS), as well as occurrences of 144 M ≥ 5.0 earthquakes in Taiwan during 1997 - 1999. Results show that the foF2 and TEC yield similar tendencies, and often concurrently register pronounced decrease anomalies 4 days before the earthquakes. A detailed investigation of anomalies appearing before and after the earthquakes confirms significant decreases in the foF2 and TEC to be the pre-earthquake anomalies.