The investment platform said the typical summer lull had not yet materialised as customers continued trading in July.
The £11bn Fundsmith Equity was the most bought fund for a 19th consecutive month, according to Interactive Investor.
Terry Smith’s flagship fund has been the most popular choice among the platform’s customers every month since January 2016.
The £10.1bn Woodford Equity Income and £2.9bn Lindsell Train Global Equity were second and third most popular funds in July among Interactive Investor customers.
Ms O’Keeffe said: “The policy of putting your name on it is working out for our top three fund managers. Although the march to passive funds appears somewhat relentless, investors are still expressing a preference for good, actively managed funds which can deliver outperformance.”
Lloyds was the most-traded blue-chip stock in the month, with AstraZeneca and GlaxoSmithKline making up the top three. Meanwhile a rally in commodity prices saw oil and mining stocks rise to popularity.
UK Oil & Gas and Sirius Mineral were the most traded Alternative Investment Market shares on the platform.
Rebecca O’Keeffe, head of investment at Interactive Investor, said better-than-expected second quarter results from many commodity producers boosted investor sentiment in the sector.
She said: “UK Oil & Gas saw its share price more than double in June and it doubled again in July, as the company piled on good news about the prospects for its well in West Sussex.”
Four of the five most-traded ETFs in the month were from the Vanguard stable, with its S&P500 and FTSE All-World trackers taking the top two spots.
Gary Millward, financial consultant at Alan Steel Asset Management, said: “The 'sell in May' mantra is something which retail investors don’t seem to pay much attention to and rightly so as there is no evidence it works, so it’s no surprise that consumers are continuing to trade through the summer months.
“Fundsmith and Woodford Equity Income are hugely popular funds and the managers have great track records, but they are multi-billion in size, so investors might pause for thought before putting more money into them and consider something smaller and more nimble.”