diff --git a/README.md b/README.md
index 18fc5bd0..2d3caa71 100644
--- a/README.md
+++ b/README.md
@@ -415,7 +415,7 @@ where possible parameters include (the first value in [*|*] is the default)
                                eg: --bc ______010 saves photons exiting at y=0
  -u [1.|float] (--unitinmm)    defines the length unit for the grid edge
  -U [1|0]      (--normalize)   1 to normalize flux to unitary; 0 save raw
- -E [0|int|mch](--seed)        set random-number-generator seed, -1 to generate
+ -E [1648335518|int|mch](--seed) set rand-number-generator seed, -1 to generate
                                if an mch file is followed, MCX "replays" 
                                the detected photon; the replay mode can be used
                                to calculate the mua/mus Jacobian matrices
diff --git a/README.txt b/README.txt
index 69d3d4ed..30cd8152 100644
--- a/README.txt
+++ b/README.txt
@@ -387,7 +387,7 @@ where possible parameters include (the first value in [*|*] is the default)
                                eg: --bc ______010 saves photons exiting at y=0
  -u [1.|float] (--unitinmm)    defines the length unit for the grid edge
  -U [1|0]      (--normalize)   1 to normalize flux to unitary; 0 save raw
- -E [0|int|mch](--seed)        set random-number-generator seed, -1 to generate
+ -E [1648335518|int|mch](--seed) set rand-number-generator seed, -1 to generate
                                if an mch file is followed, MCX "replays" 
                                the detected photon; the replay mode can be used
                                to calculate the mua/mus Jacobian matrices
diff --git a/mcxlab/mcxlab.m b/mcxlab/mcxlab.m
index 4685723c..44f02b1a 100644
--- a/mcxlab/mcxlab.m
+++ b/mcxlab/mcxlab.m
@@ -84,7 +84,8 @@
 %      If any of the 4 compnents present, they should have matching row number.
 %
 % == MC simulation settings ==
-%      cfg.seed:       seed for the random number generator (integer) [0]
+%      cfg.seed:       seed for the random number generator (integer) [default 1648335518]
+%                      setting seed to a negative integer or 0 uses system clock as seed;
 %                      if set to a uint8 array, the binary data in each column is used
 %                      to seed a photon (i.e. the "replay" mode)
 %                      Example: <demo_mcxlab_replay.m>
diff --git a/src/mcx_utils.c b/src/mcx_utils.c
index 8277f580..dfc37772 100644
--- a/src/mcx_utils.c
+++ b/src/mcx_utils.c
@@ -5460,7 +5460,7 @@ where possible parameters include (the first value in [*|*] is the default)\n\
                                eg: --bc ______010 saves photons exiting at y=0\n\
  -u [1.|float] (--unitinmm)    defines the length unit for the grid edge\n\
  -U [1|0]      (--normalize)   1 to normalize flux to unitary; 0 save raw\n\
- -E [0|int|mch](--seed)        set random-number-generator seed, -1 to generate\n\
+ -E [1648335518|int|mch](--seed) set rand-number-generator seed, -1 to generate\n\
                                if an mch file is followed, MCX \"replays\" \n\
                                the detected photon; the replay mode can be used\n\
                                to calculate the mua/mus Jacobian matrices\n\