The Sensex at the Bombay Stock Exchange fell 59 points, or 0.3 percent, to 18,646 in opening trade, this morning, on fresh selling by investors, amid the monthly expiry in the derivatives segment, and falling global markets.Later, the Sensex remained weak, and stood 45 points, or 0.2 percent in the red, at 18,659 in afternoon trade a short while ago. Stock markets in Japan, China, Hong Kong, Indonesia, Singapore, Taiwan and South Korea had lost between 0.1 percent and 2.5 percent, today. The US Dow Jones Industrial Average had ended 0.2 per cent lower in yesterday's trade.