Kilmarnock pulled three points clear of Aberdeen after a well-deserved win over the Dons at Rugby Park.
Wales celebrates his match-winning goal at Rugby Park
Gary Wales was the hero for the home side and his second half strike was just reward for his side's domination.
Dons skipper Scott Severin gifted the ball to the lively David Fernandez and the Spaniard's cross was smashed home from a tight angle by Wales.
The visitors lacked punch up front and Kilmarnock were unfortunate not to have won by a more comfortable margin.
The Ayrshire side had Steven Naismith back in their starting line-up while defender Simon Ford made his first start of the season but only lasted 50 minutes.
Goalkeeper Alan Combe, who had back surgery during the summer, was also back in the squad and started on the bench.
Aberdeen were without their influential captain Russell Anderson with Zander Diamond taking his place in the Dons defence.
The visitors started brightly but it was Kilmarnock who almost grabbed a 10th-minute goal, Jamie Langfield pulling off a point blank save to deny Fernandez.
Two minutes later, Naismith cut inside his marker and fizzed a long range strike over the crossbar.
Fernandez then had a chance at the back post following a Gary Wales corner but his header from a promising position was well off target.
A deep cross from Alan Johnston in the 25th minute saw Dons defender Michael Hart almost divert the ball past his own goalkeeper.
Aberdeen rarely threatened in the first half but James Fowler had to pop up on the goal-line to clear a Lee Miller header and Darren Mackie bazed the ball wide with only keeper Graeme Smith to beat.
A terrific cross by Johnston on 34 minutes found Danny Invincibile at the back post, but his powerful header was a couple of yards wide.
And seconds before the interval, Garry Hay drove the ball across the face of the Dons' goal with three Killie attackers failing to get a touch.
The home side continued to dominate after the break but found it difficult to break down the stubborn Aberdeen defence.
However, the breakthrough came on 69 minutes when Wales gathered a Fernandez cross 18 yards from goal and ran on to blaze a shot beyond Langfield and in off the post.
Invincibile almost doubled Kilmarnock's lead but Langfield brought off a good save to turn the ball round the post.
Kilmarnock: Smith, Fowler, Hay, Ford (O'Leary 50), Grant Murray, Frazer Wright, Johnston, Invincibile, Naismith, Wales, Fernandez (Nish 78). Subs Not Used: Combe, Di Giacomo, Stephen Murray, Leven, Adams.
Booked: Grant Murray. Goal: Wales 69.
Aberdeen: Langfield, Hart, Diamond, Foster, Considine (Daniel Smith 74), Nicholson, Dempsey (Daal 68), Severin, Clark, Mackie, Miller (Maguire 89). Subs Not Used: Soutar, Macauley, Donald, Stuart Smith.
Booked: Hart, Diamond.
Ref: S Conroy