He played his first competitive rugby for the local Clichy based Club Sportif before joining the southern giants Biarritz when he was just 17. Betsen made his first appearance for France aged 23 in 1997, as a replacement against Italy, but did not receive his second call-up until the 2000 Six Nations Championship. Thereafter he became a regular starter in the national team. Nicknamed la Faucheuse (the Grim Reaper), Betsen was regarded as one of the fiercest tacklers in the game, and was also renowned for his high work-rate on the field.