Arruti, who is a regular member on the Challenge Tour (similar to the U.S. Buy.Com Tour), leads by a stroke over Tobias Dier (66) and by two strokes over three others including Andrew Marshall, Stephen Dodd and Mattias Eliasson.
He is currently ranked 24th in earnings on the smaller tour, and knows a good showing this week could go a long way for securing a spot on next seasons regular tour.
Ive struggled over the last few weeks in the big tournaments on the Challenge Tour, but played with confidence today and that made the difference, he said.
Starting his round on the back nine first, the Spaniard and friend of countryman Jose Maria Olazabal made back-to-back birdies at Nos. 13 and 14 and then rolled in a 70-foot putt for eagle at the par-5 18th.
Then, he birdied the fifth and sixth, and finally canned an eight-footer for one final birdie on No. 9 and 65.
I played very well, very easy, Arruti said. Hit the driver well, the irons well, and only missed one shot on the seventh, but made a good up and down.
Likewise playing well was Dier, who has made a habit of shooting low scores at Slieve Russell, as his 65 and finish of fourth last year would attest.
I seem to shoot my lowest competitive rounds here, Dier said. It might be the smallest event of the year on the European Tour, but it is a lovely event and a great course.
While the course might be a great one, the weather was not so nice on Thursday.
A two-hour storm delay forced a two-hour layoff in the early afternoon, yet all players managed to finish their rounds.
Full-field scores from the North West of Ireland Open