Montgomerie, beginning one shot behind the joint overnight leaders, England's Peter Baker and Sweden's Anders Forsbrand, after a four-under- par 68 yesterday, had birdies at both the first and third holes, to go to six under.
Despite persistent rain and bogeys at both the seventh and ninth the 30- year-old Scot, helped by another birdie at the short eighth, went to the turn in 35, at which point he was joint leader.
His fourth birdie of the day, at the 345-yard 12th, put him back in front but a six at the long 17th and a bogey five at the last gave him a 72 and a two-round aggregate of 140.
This left Baker, who had yet to start his second round, one shot clear because Forsbrand, beginning at the 10th, bogeyed his opening hole to fall back and join Montgomerie at four under.
The Welshman Paul Affleck, with a bogey at the 10th and a birdie at the 15th, was also four under after six holes today but it was another bleak day for the Spaniards, Seve Ballesteros and Jose Maria Olazabal.
Ballesteros was level par for 17 holes yesterday but that still left him at three overand in danger of missing the 36-hole cut.
Olazabal, who shot a second round 74, was also three over in the difficult conditions in which only three of the early starters, David Gilford, Paul Broadhurst and Jesper Parnevik of Sweden broke 70 on this par 72 course.
SCANDINAVIAN MASTERS (Forsgardens, Kungsbacka, nr Gothenburg) Early leading second round scores (GB or Irl unless stated; * denotes amateur): 139 S Ames (Tri) 68. 140 C Montgomerie 72; R Mann 70. 141 S Richardson 71; R Karlsson (Swe) 69; P Affleck 73. 142 P-U Johansson (Swe) 70; P O'Malley (Aus) 69, B Marchbank 71; D Ray 70. 143 P Broadhurst 69; R Davis (Aus) 71; J McHenry 73; M McNulty (Zim) 74; S Grappasonni (It) 71; *H Nystrom (Swe) 71; *L Westerberg (Swe) 72; D W Basson (SA) 73; J Townsend (US) 69; D R Jones 73. 144 M Roe 74; S Struver (Ger) 73; S Little 71; 147 J Rystrom (Swe) 74; J Parnevik (Swe) 69; M Mouland 73; A Binaghi (It) 71; O Nordberg (Swe) 76. 145 T Johnstone (Zim) 74; D Gilford 69; M Harwood (Aus) 73; P Walton 74; E Els (SA) 74; H Clark 71; *C Hanell (Swe) 74; A Bossert (Switz) 73; A Gillner (Swe) 71. H Selby- Green 70. 146 R Hartmann (US) 73; O Sellberg (Swe) 73; P McGinlay 73; T Giedeon (Ger) 73; M McLean 73; J Berendt (Arg) 71; J Robinson 74.
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