Danny Lennon's career as Airdrieonians manager got under way with a defeat by Arbroath at Gayfield.
Nikolay Todorov's second-half strike, a tap-in following a square ball from Aberdeen loanee Findlay Marshall, gave the newly-promoted Championship their second win of the season.
Lennon, 56, was appointed through the week - his second spell as boss at Airdrie - after the departure of player-manager Rhys McCabe earlier in the month.
The loss means Airdrie are bottom of the table after four games, without a win in the new league season.
