ISR's Daniel Ricciardo drove a faultless race from pole position to win his second Monaco World Series by Renault race, while America's Alexander Rossi crashed out.
It was Ricciardo's second victory of the season, following his success at Monza two weeks ago, and promoted the Toro Rosso Friday practice Formula 1 driver from seventh to fifth in the drivers' standings.
Carlin's Robert Wickens chased the Australian throughout, but was unable to attempt a pass. Wickens was rarely more than 1.5 seconds adrift, and two safety car periods and a late midfield crash on the approach to the Loews hairpin narrowed the margin of victory to 0.4sec.
"I think the second time here made it a little bit more relaxed," said Ricciardo. "The race seemed to go quite quickly, whereas last year it went on forever. I wasn't too concerned about the safety cars. Robert had good speed after the restarts but I think I was faster on the longer runs."
Front row starter Brendon Hartley (Gravity-Charouz) had a disappointing getaway from the dirty side of the grid and dropped to third, but the Kiwi initially remained in touch with the leaders. On lap seven, Hartley set the fastest lap and closed in on Wickens' gearbox, but the black and gold car would overshoot the chicane on the following tour.
Careful to avoid a repeat of a drive-through penalty for a similar offence on an earlier visit to the principality, Hartley lifted off markedly. A subsequent heavy lock-up into the chicane took the bite out of Hartley's challenge and he finished third, some 11sec down.
Epic Racing's Albert Costa took fourth spot but failed to demonstrate his usual race pace. Fortec's Alexander Rossi exerted intense pressure on the Spaniard, but the American's race ended in a heavy crash at Massenet, the location of his accident in 2010.
Rossi was passing Pons Racing's Oliver Webb, who was recovering from a moment down the Ste Devote escape road, when the pair made contact. Rossi complained of a sore left foot on his return to the paddock and has requested a doctor. Webb pitted after the incident with a punctured left-front wheel.
Motorland and Monza winner Kevin Korjus put in a mature display to take fifth for Tech 1 Racing. French racer Nelson Panciatici (KMP Racing) held off Korjus' teammate Arthur Pic to complete the top six.
Championship leader Jean-Eric Vergne was one of seven drivers to receive a 10sec penalty for cutting the Ste Devote corner on a typically congested opening lap, and as a result dropped out of the points. Andre Negrao was another to receive a penalty, but the Brazilian was unfortunately not circulating at the finish. Both Negrao and his Draco teammate Stephane Richelmi retired at the Mirabeau following attempts to avoid Jake Rosenzweig's stricken Mofaz machine.
Rosenzweig's accident occurred on a restart following a safety car period. KMP Racing's Anton Nebylitskiy and Hartley's teammate Jan Charouz were the cause of the first full-circuit caution. A competitive Nebylitskiy took to the pavement at Loews as he dived inside Charouz, but the cars touched wheels on the exit of the hairpin, launching Charouz into the air.
Vergne remains in the lead of the championship on 94 points, but is now only five ahead of teammate Wickens. Costa moves one point in front of Rossi in third with 74, while Ricciardo lies fifth on 61.
Round five of the championship takes place at the Nurburgring on June 18-19, where the World Series by Renault's double-header format resumes.