Yorkshire recovered magnificently from a disastrous first hour, but then allowed Somerset to regain the upper hand by close of play when they held an overall lead of 127.
At the start of a sunny day England stalwart Andy Caddick bowled Simon Widdup and Anthony McGrath with his first two deliveries, and in his next over rapped Darren Lehmann so painfully on the right hand that the Australian had to retire hurt.
Lehmann went to hospital for an X-ray that showed nothing worse than a bruised index finger, and he later returned to play a gem of an innings which was all too short from York-shire's point of view.
Before that, Caddick continued to plague Yorkshire, who dipped to 16 for three when he found the edge of David Byas's defensive bat for wicketkeeper Rob Turner to take the catch.
Steffan Jones, having created problems of his own from the Kirkstall Lane end, dealt a further blow by trapping Craig White lbw, and Yorkshire were 35 for four.
But they were transformed by an exhilarating stand of 125 in 37 overs between Michael Vaughan - showing all his England skills - and Gary Fellows, who bravely went for his shots from the start and equalled Vaughan with the quality of some of his strokeplay.
Quiet by nature, Fellows is fearless at the crease and his adventurous approach helped him to prosper while Vaughan seemed undismayed by all that had happened previously.
Vaughan went to his half century shortly before lunch, and Fellows achieved his maiden 50 soon afterwards, but having advanced to 63 he fell lbw to Graham Rose.
Lehmann reappeared to take a heavy toll of Rose, and had plundered eight boundaries in his 38 from 37 balls when he drilled Caddick to square leg and Yorkshire's hopes of gaining a first-innings lead began to disappear.
Richard Blakey was lbw to Jason Kerr without scoring and Vaughan's 65-over vigil was ended by a fierce drive at the tireless Caddick, which was well caught low down at cover point by Michael Burns.
Piran Holloway and Marcus Trescothick began Somerset's second innings at a rush after tea.
The 50 stand between the openers contained ten boundaries, and it took Lehmann's left-arm spin to part the pair at 63, Trescothick driving back his second return catch of the match.
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