Voice Activity Detectors (VAD's) are widely used in speech technology applications where available transmission or storage capacity is limited (e.g. mobile, DCME, etc.) and must be utilised with maximum economy. Modern day digital speech coding algorithms can provide toll quality speech at bit-rates as low as 8kbit/s (e.g. ITU-T G.729) and the use of a VAD can achieve further economy in average bit-rate. This paper presents a modified version of the GSM VAD, for use with the ITU-T 8kbit/s speech coding algorithm CS-ACELP, which makes an active/inactive decision for every 10 ms coding frame. The performance of the proposed voice activity detector is compared to that of the GSM coder in terms of VAD errors and subjective quality. Results indicate that the modified VAD has similar performance to the standardised GSM VAD while operating with G.729 parameters and coding frame size.