Isaac Thanks for the several alternatives. They work very well! Jesus

Isaac, In your code for finding specific derivatives

specificD = Position[ eqns,#]& /@ { _'''}      (* Find positions. *);

the result is a list of positions. How can I use them to extract the actual derivative e.g. x'[t]. The positions are lists and when I give them as arguments of [[...]] the result is not the expected. This may be a silly question, sorry!

(* Mathematica 7 *)
(* Clear memory *)
Clear["Global`*"];
(* Define a list of equations. *)
eqns = {x == 2y, x'[t] == 2t, x''[t] == 4t +1, y'[s] == 2z, y == t, y[m] == 2m, g[t] == x'[t]}


(* To find all equations with any derivative. *)
anyD = Position[ eqns, Derivative] (* Find positions. *);
pos  = Take[#, 1][[1]]& /@ anyD    (* Isolate first "address" element. *);
disp = Take[ eqns, {#}]& /@ pos    (* Print out all eqns with any derivative. *)

(* To find equations with a specified derivative. *)
specificD = Position[ eqns,#]& /@ { _'''}      (* Find positions. *);
pos       = Take[#, 1][[1]]& /@ specificD[[1]] (* Isolate the "address" element. *);
disp      = Take[ eqns, {#}]& /@ pos           (* Print out the equations. *)