This paper examines variations of the greatest plasma frequency in the ionosphere, foF2, recorded by the Chung-Li ionosonde (25.0° N, 121.1° E) before M≥6.0 earthquakes during 1994–1999. The 15-day running median and the associated inter-quartile range are utilized as the reference and the upper or lower bounds to monitor the ionospheric foF2 variations for finding seismo-ionospheric signatures (precursors) of the earthquakes. It is found that precursors, in the form of the recorded foF2 falling below its associated lower bound around 1200–1700 LT, appear 1–6 days prior to these earthquakes. On September 20, 1999 UT (September 21, Taiwan local time) a large Mw=7.7 earthquake struck central Taiwan near the small town of Chi-Chi. We analyzed the foF2 and found three clear precursors 1, 3, and 4 days prior to the Chi-Chi earthquake.