ST Briavels opener Will Peters scored an undefeated 62 in a game which saw a combined total of less than 200 for both sides.
Visitors Penallt and Redbrook came to the Wye Valley derby undefeated in the Cheltenham, Gloucester and Forest League this season and got off to a cracking start.
The Division One leaders’ captain, Chris Margetts removed the wickets of Darryl Reynolds and Stuart Williams with the score on 19 and then also bowled Geraint Reynolds with just four more runs added.
A partnership of 53 between Peters and Phillip Seabury inched the home side towards the three-figure threshold.
Rajesh SR picked up two wickets in 3.3 overs in a spell cut short by a shoulder injury
Matthew Spoor also claimed a wicket as St Briavels were limited to 106-6 in 30 overs in a game which started late because of the morning rain.
In the field there were two catches for Kamaal Mitha – with one requiring a full-length dive – and one each for Myles Ruck and Cameron Pardington.
The first Redbrook wicket also fell on 19 with Rob Cole bowling Graham Emerson.
But those runs had come at a slow rate, taking 10 overs as bowlers Stu Williams and Rob Cole kept Redbrook penned in.
Emerson was replaced by Spoor who put on 32 runs for the second wicket to take them to 51-2.
But Margetts, who top scored for the visitors with 20, and Liam Hurcombe went in quick succession to reduced Redbrook to 51-4.
In the 25th over, Williams struck twice as the visitors went from 77-5 to 83-7.
Redbrook were 24 off the target but a wicket in each of the next two overs gave St Briavels a 20-run victory.
Despite suffering their first defeat of the season, Redbrook remain top of the table.
St Briavels move into third place, leap-frogging Corse & Staunton Thirds, whose game at Upton St Leonards was cancelled.
Bream remain second despite being defeated at home by Cinderford St John.