The index opened at 7,211 on Tuesday, up 233.06 points, in its first session since Friday after Monday's substitute holiday for Liberation Day.
Decliners outnumbered advancers 630 to 234, even as the large-cap KOSPI 200 rose 4 percent. The divergence highlights the narrow nature of the rally, with gains concentrated among a handful of large-cap stocks while most shares declined.
Foreign investors have concentrated their buying in a narrow set of names. Over the week to Friday they bought a net 6.75 trillion won (US$4.78 billion) in Seoul shares, of which SK hynix took 2.45 trillion won ($1.73 billion) and Samsung Electronics 2.29 trillion won ($1.62 billion).
Individuals sold a net 7.40 trillion won ($5.24 billion) over the same week, taking profit into the advance.
SK hynix traded at 1,786,000 won ($1,264.87), up 141,000 won, a gain of 8.6 percent. Samsung Electronics traded at 287,500 won ($203.61), up 13,000 won, or 4.7 percent.
SK square, the holding company whose main asset is its SK hynix stake, rose 8.9 percent to 1,257,000 won ($890.23).
The semiconductor sector gained 6.3 percent, the strongest industry group on the main board. Shipping followed, with HMM up 6.4 percent.
Outside the chip complex, the market was red. Among the largest listed companies, Hyundai Motor slipped 0.2 percent to 452,000 won ($320.11), LG Energy Solution fell 2.8 percent to 359,000 won ($254.25), and Samsung Biologics lost 1.7 percent to 1,522,500 won ($1,078.26).
Foreigners bought a net 904.0 billion won ($640.2 million) in the opening 40 minutes, against 917.6 billion won ($649.9 million) of individual selling.
The advance did not reach smaller names. The KOSDAQ slipped 0.3 percent to 862.30.
Seoul is also moving against the overnight trend. The Dow Jones Industrial Average fell 0.5 percent and the Nasdaq Composite lost 0.3 percent on Monday in New York. Traders viewed Tuesday's gains as a catch-up to last week's strength in memory chip stocks rather than a fresh signal from Wall Street.
The main index in Seoul has now risen more than 20 percent from its late July low, marking what is generally considered a technical bull market.
In Tokyo, the Nikkei 225 traded at 69,025.70, down 0.3 percent.
The won traded at 1,412 per dollar, stronger than Friday.
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