Michael Appleton's reign as Portsmouth manager got off to a disappointing start with defeat at Watford.
The former West Bromwich Albion coach, who took over at Fratton Park nine days ago, could not lead Pompey to a first away win of the season.
It was a deserved victory for Watford, who opened the scoring when on-loan Michael Kightly fired in his first goal in two-and-a-half years from 25 yards.
Marvin Sordell's superb volley from a Mark Yeates corner sealed victory.
Appleton's side went a goal down inside two minutes.
Former England Under-21 international Kightly, on loan from Wolverhampton Wanderers, found space on the left flank and put the home side ahead with a speculative drive that beat Stephen Henderson from long range.
The winger had a further chance to score when his effort hit the side netting, but Pompey were starting to impress in possession - and only a last-ditch challenge from Nyron Nosworthy thwarted visiting striker Luke Varney when clear on goal.
It was a chance Portsmouth were left to rue as they soon went two goals down, Sordell connecting exquisitely with Yeates' corner.
Sean Dyche's men continued in the ascendancy after the break, Sordell missing a clear-cut chance for the game's third goal in the 69th minute when he flicked a header over from close range.
With 10 minutes remaining, Varney's header forced Scott Loach into his first save of the match.
The keeper was again called into action to thwart a Erik Huseklepp free-kick in the 90th minute, but that was as good as it got for the 1,727 visiting fans.