Leinster (9) 33
Tries: Jackman, O'Driscoll, Keogh Cons: Sexton, Holwell 2 Pens: Sexton 4
Castres (0) 3
Leinster skipper Brian O'Driscoll is tackled by Gideon Lensing
Leinster secured an expected Heineken Cup win over Castres but failed to gain a crucial Pool Two bonus point.
The Irish province dominated possession but were just 9-0 up at half-time through three Johnny Sexton penalties.
Leinster landed their first try soon after the break through hooker Bernard Jackman and Brian O'Driscoll got in for the second try in the 66th minute.
Replacement Simon Keogh managed a late third but Leinster ran out of time as they pressed for a bonus-point score.
Leinster, six points clear in the group, now face Castres away on Friday before the expected crucial showdown away to Wasps on 17 January.
By that stage they should have a better idea about the cost of their failure to bag a fifth point at the RDS.
The Magners League champions will certainly feel they should have had a fourth try as they launched a series of attacks on the French club's line.
Leinster really did not get into their stride in the first half and had to rely on the boot of St Mary's fly-half Sexton for their interval advantage.
The Irish side got a break in the 47th minute when Shane Horgan enjoyed a favourable bounce from his kick allowing him to feed Jackman who charged to the line for the first try.
Further Leinster pressure was rewarded when Ireland skipper O'Driscoll got in on the left for the second try in the 66th minute.
It looked like Michael Cheika's men had the platform to push for a bonus point.
They did get a third try when replacement Keogh finished off a surging move in the 76th minute, but a fourth touchdown eluded them.
Leinster: Dempsey; Horgan, O'Driscoll, Fitzgerald, Kearney; Sexton, Whitaker; Wright, Jackman, van der Linde, Toner, O'Kelly, Elsom, O'Brien, Heaslip.
Replacements: S Keogh for Dempsey (70), Blaney for O'Driscoll (72), Holwell for Sexton (62), Keane for Whitaker (68), Healy for van der Linde (70), Hogan for Toner (68), S. Keogh for O'Brien (70).
Castres: McIntyre; Sika, Kefu, Mazars, Carballo; Legardere, Senio; Lensing, Bonello, Ducalcon, Michaux, Gaston, Tomiki, Nell, Faure.
Replacements: Bouquie for Carballo (16), Forestier for Lensing (62), Giorgadze for Bonello (56), Malonga for Nell (28).
Not Used: Staibano, Tekori, Tillous-Borde.
Ref: Peter Allan (SRFU).